To be fair, when WEG started publishing there wasn't much material to work with.
To be further fair, the whole Expanded Universe owes a TON to the work WEG put into adding detail to the Star Wars Universe.
When Lucasfilm decided to greenlight writing the first novels, Timothy Zahn decided to pull from the WEG material whenever he needed a world or vehicle or species because it was work that had already been done. Timothy Zahn even released a campaign setting DarkStryder for WEG Star Wars, further tying the two together. The result was that WEG was literally the foundation from which all of the Expanded Universe was built upon.
In turn, the success of the Expanded Universe is, from accounts, part of the reason that George Lucas decided to do the prequels. There were other factors, like technology finally catching up to his vision, but the financial success of the EU was what convinced him that there was still enough interest in Star Wars to start working on the prequels.
Of course George completely ignored the EU other than a few choice names (ex. Coruscant and its "city-planet" nature came right from Timothy Zahn) and that was, honestly, the start of the problems for the EU as core elements of their established canon had to be thrown out and/or retconned to match the prequels. They also had to trash whole storylines because of edicts from above... Anakin Solo was killed off precisely because George felt it would cause confusion to have two characters in Star Wars named Anakin; so even though Anakin was set up to be the hero of the New Jedi Order trilogy who would redeem the name Anakin, he got unceremoniously ganked midway through and the entire rest of that series was a scramble to recover and further resulted in a cascade of other changes that resulted in such idiocy as "60 is the new 30" to explain why Luke, Han and Leia were still running around as the main protagonists because they'd killed off 90% of the younger generation due to one edict or another.
Frankly, Star Wars is largely the mess it is today in part because the success of WEG and Timothy Zahn enjoyed in reinvigorating what was then a mostly fondly remembered, but no longer anything like a cultural zeitgeist, film series and everyone and his uncle (including Uncle George himself) deciding to put their stamp on the Star Wars Universe (even The Mouse... which did started its stamping by stamping out everything except the actual films and The Clone Wars animated series).