The benefit is being able to display 3+ different ads on rotation that change every minute or two. That, and labor is cheaper when they're not 50ft off the ground

Labour is expensive

I dunno how often they normally change, week or so?

Quick bit of googling suggests printed billboards have a ~$1k startup cost to the advertiser then a flat rate monthly fee, so I'd hazard a guess its probably 3-6 months at a time to amortize the startup cost