One reason you might want to make a sprite instead of a Quad is because making a quad is more difficult. Blitz isn't a language only for pros who know how to transform points and construct meshes from scratch. :-)
Another reason is for z sorting. If you have objects with alpha in your scene, like trees, then if all your quads are in one mesh, then they will all use the same point (the center of the mesh) to dtermine what z order to draw in. So they'll all draw in front of certain trees and all draw behind certain other trees.
But if you have srpties then they're all seperate objects and will each use their own center to dtermine draw order.
Plus it's easier to fade them out as needed as a sprite, though if you're clever you can fade them out with vertex colors so that's not a major reason to use them.
Another reason is for z sorting. If you have objects with alpha in your scene, like trees, then if all your quads are in one mesh, then they will all use the same point (the center of the mesh) to dtermine what z order to draw in. So they'll all draw in front of certain trees and all draw behind certain other trees.
But if you have srpties then they're all seperate objects and will each use their own center to dtermine draw order.
Plus it's easier to fade them out as needed as a sprite, though if you're clever you can fade them out with vertex colors so that's not a major reason to use them.