There's more, perhaps more relevant in countries with more than 2 mainstream parties. There's a "hey this is a viable idea, look at how many people are supporting it" kind of thing.

For example in local elections in the UK, plenty of people put up a little poster thing for their candidate. I definitely had my attention drawn to candidates I considered no-hopers and gave them a second look. I wouldn't have if I thought they have no support otherwise.

I think this is a bit different to simply repeated advertising. There is also a component of "join us, we're a cool community [of people voting for X], it's OK". Think about, more so in the past, people were often reluctant to admit they are voting for a populist candidate. Stickers can bring such "uncool" candidates into the mainstream.