It can take some of the "tinny" sound out. Depending on the style of the lug, some can be more noisy than others.
That having been said, most of the manufacturers( the big 3 ) didn't do that 100% of the time.
I have a Super Sound King COB that had white felt just on the back wall of the lug so to speak and yet I've got a Hollywood Ace Chicago vintage, that didn't have felt and lastly, the Student RK snare I'm working on had felt wrapped all the way around the spring.
I know that most people wouldn't do this, but I have wanted to play a snare with the felt and then without it and see if you can really hear the difference.
My guess is that depending on the drum and lug styles you may be hard pressed to notice.
I basically err on the side of if I think the drum hasn't been tampered with and if it had felt in it, I put it back in and the converse holds true if it didn't.
I'm sure there are members with more input and experience that may solidify a more definite opinion based on having worked with more drums.
Good luck!
BGT