Standard tuning is an invention of necessity. It allows the best compromise between comfortable fingering for chords and comfortable fingering for scales. If guitars were tuned to 5ths like most stringed instruments, scales would be really hard to play because of the angle you have on the guitar compared to like a violin or cello. If guitars were tuned entirely to 4ths then chords would be a bit of a mess and you wouldn't have a nice octave thing going on with both the highest and lowest string tuned to E. Hence the major 3rd interval between the G and B strings.

Standard tuning first appeared in Italy in the 1500's, when five course chitarra battentes were tuned ADGBE. These replaced four course guitar-like instruments which were tuned closer to how ukuleles are today. When a sixth course was added in the mid-late 1700's E was the obvious choice. A 'course' is two strings next to each other by the way, so a modern 12 string guitar is a six course instrument. A chitarra battente would have been tuned the same as a modern 12 string guitar, minus the low E strings.

Spanish tuning as a nickname for open G is thought to have originated in America with British expatriate Henry Worrall, and his tune "Worrall’s Original Spanish Fandango", played in open G tuning. Worrall copyrighted the composition in 1860 and it quickly became a standard parlor tune. It was so popular that it was likely included in small tutorial books that would be packaged with guitars at the time. In this way it influenced the folk, country, and blues of the early 1900's. Here is a country rendition of Spanish Fandango from the 1930's, with the melody played on a steel guitar.

Hi there -- while there's always more to be said on these topics, we had a previous thread here about guitar tuning that may be of interest.