Logos provide easy to remember, identifiable ways to show affinity to a political candidate or cause, just as they do to a product, sports team, etc. Think of how many logos we remember from our childhood; "No matter what shape your stomach's in", " my bologna's spelled...", "DYNOMITE!!!", "All the Way with LBJ!"
Political branding continues to be important. Look at MAGA. I daresay there isn't an adult in America who doesn't recognize it and almost all understand what it stands for. As far as effective brands, I don't think recent brands are as effective as those in the 1900's. Those brands were much more uplifting and brought out positive, rather than negative feelings.