how did the guitar change the course of western society?

The usual answer to this question is that the guitar supplanted the piano as the “parlor” instrument, that is, the instrument that ‘everyone’ learned to play for entertainment and general demonstration of ‘having a talent’.

From the Victorian period on, a sign of wealth and good breeding for a family was shown by owning a piano. Young ladies in the family learned to play, at least well enough to hold their own through one or two pieces at a party, perhaps even well enough to play accompaniement to popular songs that everyone could sing.

Around the middle of the 20th century, almost every house in the country had a spinet or small upright piano in the parlour, that room reserved for visitors to sit and chat, and young people to do their courtin’. Again, the young folk would be forced to take lessons so that they could play at least enough to ease a party a bit: this was an established habit from well before the Radio. Around the 1950’s, the guitar started to become ‘the’ instrument, instead of the piano.

The piano’s big benefit was that it was largely mechanical: as long as it was kept in tune (by a specialist piano tuner), pressing a key made a note, and very little real musical talent was required to produce the notes, unlike flute, violin, or other instruments where the player has to practice long hours before they can play in public. So the major job was to learn to make your hands coordinate enough to press the right keys at approximately the right time.

The guitar was similar: the chords were easy to learn, strumming was not rocket-science, and it had two additional benefits: it was portable (so you weren’t limited to one room) and an increasing body of popular music was being produced that could be ably accompanied with it.

With the 50’s came Rock music, which primarily employed guitars and drums. By some reckoning, the guitar was the ultimate liberator: anyone could get a guitar for fifty-sixty (maybe a hundred) dollars and start playing overnight, and start a band, with the hopes of becoming a star! (And for some, it even worked!)

Interestingly, although outside of your questions usual boundary, the guitar’s popularity is actually an echo of an earlier time: During the Renaissance (1550 to 1650), the Lute occupied a similar position: most people of breeding would learn to play it, and it was a popular, portable, able accompaniment for songs. Barbershops would keep a lute so customers waiting to be shorn could play it.

In many ways, the guitar ‘revolution’ returned this kind of populism to music, taking it from the hands of professionals and putting it into the hands of ‘common people’.