generally, what would they generally keep watch against ?

Where bears and other animals a bigger problem than thieves or brigands ?

"Converse" question: Where or what routes were typically ones that were well enough developed that travelers didn't need to "camp out" but could stay in ... I don't know what... Inns? Hostels?

So once you eliminate that, what would be examples of routes where there would be non-military groups traveling, where they would need to "set up camp" and then potentially keep watch at night?

(I guess "who" was traveling would also be an issue - were there traders who traveled frequently? Am I right in understanding that some Medieval monks and clergy of the Catholic Church traveled frequently and extensively? Is it accurate to infer that the vast majority of the population of Europe spent their lives close to where they were born and never did multi-day trips, or is that not true?)

I think this question will be hard to answer unless you narrow it down some more. Might I suggest specifying the time period a little more precisely, and perhaps a specific culture?

It's anecdotal, but I can think of one episode that answers “yes” to your question about night watches though it’s couched in terms of a ghost story. In a collection of such tales from Byland Abbey in north Yorkshire, c. 1400, the beginning of one story has this detail:

It is worth remembering that there was a man from Cleveland named Richard Roundtree who, leaving his pregnant wife behind, went on pilgrimage to St. James at Compostela with a number of other people. One night they spent the night in the woods near the royal road. Here one of them kept watch for part of the evening for fear of the night while the others slept securely.

Source: J. Shinners (ed.) Medieval Popular Religion, 1000-1500, 2nd ed. p. 260.

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