For a lot of people it's also about keeping their culture present in public.
I don't so much care to agree with it, but I don't have a repressed culture with it's own language, so I'm undoubtedly biased.
But research a little into the Welsh, Irish and Cornish languages and you'll see how much it can mean to some people just to have a little slice of their culture out there.
And having a sign with a sentence on it two different languages can do that. It's not a major price to pay. (Even if it's stupid that I get posters and leaflets sent to me about being environmentally friendly, and they happen to be double-sided or twice the size in order to accomodate English and Welsh).
And then imagine you're a hispanic US citizen, and you and your culture make up a big part of the population. You're not a small slice. And yet you don't see your language written down much on signs and things.