It makes some sense for small local election candidates in terms of actually gaining votes. It's not like they are really on TV or household names like the larger or obviously presidential elections. These signs are to get some exposure so people might remember your name and 3 word slogan when the ballot is in front of them. They probably throw away that mail flyer you sent but they will remember the sign slogans.

The reason you see these signs in the bigger elections is largely an American phenomenon (but definitely not exclusive).

For a long time now many Americans VERY strongly associate their own identity, morals and beliefs based upon political affiliation. aka. Identity politics. This is not true of many other countries.

Placing these signs flies the flag of your tribe against your enemies. It's to hang the banner of your camp and display your factions army size in the current war. It's not to convince others to change their political opinion. It's to rally your own troops.

It's purpose is to feel personally strong and proudly display your identity. To raise your visor and say "This is who I am" to all the other passing knights, be they friend or foe.

If your party wins then you triumph over your enemies and you feel like a great battle has been won and you were a solider in that battle. You flew the flag. You had the bumper sticker. You "fought."

The personal satisfaction and pleasure gained by this mentality is clear. You have to really liken it to winning an actual war. The more you participate in the war, the more satisfaction you are likely to feel.

The smug victory over your defeated foes. You want them to know that YOU personally defeated THEM. Because that is your identity.