It’s now pretty well known that West End Games’ take on Star Wars became a major seed of what became the Expanded Universe. Whilst additional stories of questionable canonicity have always been part of the franchise – Alan Dean Foster did Splinter of the Mind’s Eye back when the original trilogy was coming out, based on the story George Lucas had mapped out for the second movie in case the studios wouldn’t give him the budget for Empire Strikes Back – but it’s fair to say that the whole Expanded Universe thing didn’t kick into high gear until Timothy Zahn’s Thrawn trilogy, and it’s well known how when he was writing that Lucasfilm gave him a fat stack of West End supplements to use as background reference material. Although much of the Expanded Universe has been declared non-canon by Disney (though they still acknowledge its existence under the Star Wars Legends label), extensive details of the West End line remain having crept into canon via the later films and other materials that didn’t get made into unhistory.
One reason that the West End line has been so influential is because of the sheer mass of material produced for it. As well as providing sourcebooks based on obvious subjects like the Rebellion and the Empire, it also did supplements based on specific releases, so not only did you have supplements focused on each of the original trilogy but you also had a phenomenon where each new Expanded Universe hit ended up getting its own West End sourcebook building on what it did, and since that Expanded Universe stuff was building on things West End had done you ended up with a feedback loop going where West End were constantly churning out ideas. (This was exacerbated in their late-life shovelware period, where they cranked out Star Wars stuff at a wild pace because it was a licence to print money for them and the main thing making their business viable.)
Just as West End was the wellspring of the Expanded Universe, The Star Wars Sourcebook is the seed of that approach. The actual 1st edition Star Wars RPG rulebook didn’t actually include an awful lot in the way of setting information, and to be honest it didn’t necessarily need to – if there’s one franchise out there where you can reasonably be sure most people have a passing familiarity with the setting, it’s Star Wars. The Sourcebook was published alongside the core rules and was mainly authored by Bill Slavicsek, the line editor for the Star Wars RPG, and you can sort of see it as the other half of the originally intended core line. (Remember, the supplement churn didn’t go into high gear until the RPG started selling in a big way.)