Driving home from dropping off my son this morning, I noticed a zillion political signs dotting the landscape and had to wonder "what's the point?". I mean, it's not like a sign is big enough to provide any real useful information. In fact, I can only see a few possible scenarios where they'd affect voting outcome:
Sparking interest: This is the optimist's scenario -- that upon seeing a sign, the viewer will be inspired to hop on the internet and learn more about the issue or candidate. Political signs have been around for far longer than the internet, however, and it's still difficult to find anything of substance on local candidates.
Recommendations: I suppose if you saw a political sign in front of the house of someone you knew and respected, you might take that into account when making your decision. It seems equally likely, however, that the person putting up the sign might be widely viewed to be a bumbling idiot, so from a political standpoint it's likely to be a wash.
Lowest common denominator: This seems to be the likely reason that folks put up political signs -- that a significant number of voters go to the polls without any real knowledge of what they're voting on and will therefore pick a name that they recognize or a position that they think is "popular". What a way to pick a candidate.