The most common use near where I live is protecting signs, lights, or electronic equipment from bird poo. Second most common is protecting whatever is underneath the spikes, like car parking, pedestrian paths, or windows. You see them a lot in train stations protecting the electronic signs so they stay readable, or anything else hung from the roof which people might stand under.

Generally birds have plenty of other places to rest where they won't be inconvenient to humans. I've never seen anyone cover an entire rooftop with spikes.