its because usually the disc information; song artist etc were usually on the inner part of a vinyl record in a small dis right in the centre. and if you kept pullin yer record to see what one it was,in and out then it would pick up carbon or might get scratched. so it was just really a way of letting ye know what was on the record
So that you can see the title of the record on the label without handling the vinyl. It was also cheaper to produce a standard sleeve that could be used for any title, rather than the individually printed ones of latter years.