Modern-day billboards are as disposable as the paper they're printed on. They go up, they remain there for as many days or weeks as the advertiser has paid for, and then they're either ripped down or papered over for whoever's taken out the next lease on the advertising space. Back in the old days, they used to be a little bit more robust than that. If you wanted to get rid of an old-school advert, you'd have to dismantle or cut down the entire thing.
The retro market loves it when an old advert that hasn't been damaged by removal appears. Coca Cola adverts appear to be especially collectible, with the most valuable example changing hands for more than $117,000. Beer signs also frequently go for four figures instead of three. If you pass a store and you see one either rotting away or in the process of being taken down, try to persuade them to hand it over to you instead.