The catch is, you still want that water supply to be dependable when it is an old roof as well. Bird poop and the like means a filtration system.
Definitely some filtration is in order, but you can divert the first part of the rain in order to “wash” the roof, then collect the remainder as good water.
This divert wouldn’t have to be manual if you set up your system right with a vertical divert tube that fills up with water for discard and the overflow going to your collector.