I'm sure there are others on here who specialise more closely in stagecraft than I, but I can offer a few insights here. This is a great question which requires a few different perspectives to give a good answer.

 

The first bit of context I would like to attach here is that of the Elizabethan play-going scene. While there were indoor theatres and court performances which would cater specifically to a higher class of patron, the playhouses in London famously took all comers. They were also seated in an environment where a fairly large variety of entertainments were on offer on the South side of the river, some more savoury than others. Brothels, animal baiting, and plays could all be enjoyed a stone's throw away from one another.^1 Fighting and duels were also commonplace - tournaments were held at Smithfield, and public fencing contests between students of London's fencing schools were held in playhouses, open fields, and inns and taverns.^2 Public fighting was very much part of the entertainment scene, so in many ways it isn't surprising to see duels and swordfights appear on stage.

 

As for your question about whether the sword fights were 'for show', that's harder to answer. Elizabethan play-goers were certainly accustomed to sword fighting to some degree, especially given that fencing contests were potentially a part of their entertainment diet, and that it is reasonable to assume that a majority of able-bodied men at this time had some familiarity with swordplay themselves ('stab them with the pointy end!'). We do not know whether sword fights on stage generally attempted to appear convincing and real, or whether stage fighting had its own conventions without any expectation of verisimilitude.^3 Few contemporary sources discuss this at all, let alone tell us whether these stage fights were realistic. We do know that some actors, like Richard Tarlton were talented fencers, but have no way of knowing whether this was a talent that was put to use on stage to enhance realism.^4 It is worth bearing in mind that we do know that animal blood was more than likely used on the stage to produce realistic special effects, so there must have been some expectations of realism from the stage in other areas.^5

 

Now to come to Shakespeare. Charles Edelman sees Shakespeare's substantial use of on stage fencing/swordfighting as particularly unusual for the period. He says;

However prevalent the tradition of fencing in the playhouse may have been at the time Shakespeare began his career as a dramatist, it borders on the remarkable that he was the first to exploit this tradition to any significant extent. While stage comba certainly existed before Shakespeare, it is also true that most poets, at least as far as can be determined from extant texts, preferred much of their fighting to be done offstage.^6

Quite a few contemporary plays call for both blood and gore in copious quantities (Marlowe's Tamburlaine plays come to mind, which includes a character braining himself to death on stage), but few combine swordplay and dialogue in the way that Shakespeare does, which is what Edelman regards as the true innovation taking place here.

 

I think that covers the key points of cultural context for the staging of swordplay, but I'll spend a little time going into a few specific examples of the way in which Shakespeare makes swordplay an integral and symbolically meaningful part of his drama. Romeo and Juliet is probably the most oft-discussed example of Shakespeare's focus on swordplay. For example, Old Capulet calls for his long sword during the brawl in act 1 sc 1, marking him as something of an old fuddy-duddy compared to the young men with their modern rapiers. Baz Lurhmann does a great job with this in the film adaptation, where Capulet reaches for a large, old fashioned shotgun [edit: thanks for correcting me /u/g2petter, it's actually a modern pump-action shotgun he goes for], where all the young men have flashy pistols. The generational divide is exemplified here by his choice of weaponry.

 

Later in the play, the fight between Tybalt, Mercutio and Romeo features a number of jibes about the way that Tybalt fences 'by the book', and this indeed refers to a popular genre of fencing manual (The British Library has an interesting copy online here). It can be argued that this very detailed fight scene was a good opportunity to show off some fashionable continental duelling styles on stage.

 

The verbal run-down of the fight is so detailed that Adolph L. Soens reconstructs the fight and the stereotypical assumptions that come with each style of fighting that the men adopt. He describes how Tybalt is represented as 'foreign and affected' compared to Mercutio's representation as a 'rough, honest, downright Englishman' in terms of their clothing and manners, and how Tybalt's mechanical style of Spanish fencing overcomes Mercutio's Italian style (on which much contemporary English fencing was based). We have a clear set-up for 'good guy and bad guy' here, and welp, the bad guy wins.^7

 

Other plays that I don't have time to discuss in detail today (sorry) but which use sword fighting in a similarly narrative way include Hamlet, King Lear, and Macbeth.

 

To attempt a conclusion - yes, to some degree sword-fighting was included in plays because it was already part of the entertainment scene and known to be popular. Early modern plays were a visual spectacle, often full of blood and gore and it's not surprising that we see it take place on stage, especially as fencing manuals were becoming popular in print. However, the way in which Shakespeare gets a lot of his fighting done on stage and the way he integrates it into his overall symbolic landscape for the play is pretty unusual. Hope that answers at least some of your question, and look forward to see what else others with a closer degree of specialism in the field have to offer!

 

^1 Philip Henslowe, the owner of the Rose Theatre (where many of Christopher Marlowe's plays were staged) had business interests in all three of these areas, with one of his theatres set up to accommodate both plays and animal-baiting. See here

 

^2 McElroy, Mary, and Kent Cartwright. "Public Fencing Contests on the Elizabethan Stage." Journal of Sport History 13, no. 3 (1986): 193-211. http://www.jstor.org/stable/43609212.

 

^3 I recommend reading the introduction to Charles Edelman's Brawl Ridiculous: Swordfighting in Shakespeare's Plays (Manchester: Manchester University Press, 1992) for a more detailed discussion of this.

 

^4 Thomson, P. 'Tarlton, Richard (d. 1588), actor and clown' in Oxford Dictionary of National Biography. Ed. Retrieved 15 Aug. 2018, from http://www.oxforddnb.com/view/10.1093/ref:odnb/9780198614128.001.0001/odnb-9780198614128-e-26971.

 

^5 Farah Karim-Cooper and Lucy Munro, Shakespeare’s Globe Theatre History Seminar Stage Blood: A Roundtable, 13 July 2006. See here (opens pdf).

 

^6 Edelman, Brawl Ridiculous, p.11.

 

^7 Soens, Adolph L. "Tybalt's Spanish Fencing in Romeo and Juliet." Shakespeare Quarterly 20, no. 2 (1969): 121-27. doi:10.2307/2868996.

 

edit: formatting derp