First off, Sputnik 1 was when space travel went from being "just a theory" to being an actual thing. The difference is huge, do you not see a lot of theories about future technologies here and there? Can all of them be true? No matter people where sceptical to the concept of space travel.
Secondly, the successful launch saved Korolev's ass. The head designer og the Soviet Rocket program had great visions for space. However, the military wanted missiles, and nothing but that. (even ICBMs where questioned for their actual usefulness as weapons). One can truly say that he went quite far, launching the last attempt to get Sputnik into orbit after he got an order from Moscow to "stop it". When the satellite was successful, everything was of course forgiven. Finally getting the space program a budget in the Soviet Union, can indeed be said to have been the start of the amazing technological progression that was the spacerace.
Nothing is guaranteed to work until you test it!
A for-real test of an orbital rocket is an experience that can not be replaced. If that is not what you are interested in, and strictly mean the probe itself, it was indeed a simple design, featuring very limited instrumentation. Apart from "the components used did not break", the role of the Sputnik 1 was merely to pave the way for the following series of much more sophisticated Sputnik satellites.