I can think of two reasons for political stickers and posters, neither of which is among the ones listed in the article linked here. 1. Putting them up cements

I can think of two reasons for political stickers and posters, neither of which is among the ones listed in the article linked here. 1. Putting them up cements the loyalty of the person putting them up. You're less likely to consider switching away from the Purple Party to the Orange Party when you've already spent a lot of time and effort putting up Purple posters and when you've publicly declared your affiliation by plastering your property with Purple stickers. 2. Having a lot of them up demonstrates that your party has enough support to be worth bothering with. Few people like the idea of voting for a likely loser, especially in places where elections are first-past-the-post. So having lots of Purple partisans put up Purple posters lets everyone see that Purple are in with a chance and therefore not feel as if a vote for the Purple candidate would be thrown away. #1 is a reason why parties should produce these things, though not a reason why individuals should put them up. #2 seems to me a more plausible reason for putting them up than the ones in the article. If my theory about #2 is right, we might expect to see more political posters in places whose electoral systems make votes for minority parties less valuable. I don't know whether that's actually so. [For the avoidance of doubt, "Purple" and "Orange" here are arbitrary labels and aren't intended to suggest any actual parties or movements that might use those colours.]