I just don't see that there is any possible benefit, in the case of presidential elections anyway. I don't think there are many, if any, registered voters who haven't heard of Donald Trump or Joe Biden, so the goal can't be to raise awareness. There isn't any need to raise awareness of something everyone is already aware of.
I also find it extremely hard to believe any undecided voters could be persuaded either way just because they saw a sign with a couple names on it. I believe many are even given away for free, so it can't just be fundraising method. I get some of the appeal for the sign's owner; it's virtue signalling to his or her friends and neighbors, which inherently feels good. I don't understand what the candidate on the sign gets out of it though.
Since every candidate ever uses them, I know I must be missing something, but I can't figure out what.