They are a gimmick I agree, or at least not necessary. That said, there are benefits, not least the chance to for clubs to sell shirts with lettering and numbering, which must add a bit to income. Generally, I don't think it matters over much with your own team, since you usually know who is who without numbers most of the time, but with other teams, and particularly in the lower divisions where players aren't as instantly recognisable, it's easier to keep track of who is who with squad numbers and names on the back.
I guess, in truth it's really just a case of aping soccer.