Why?Because is gets into the plaster and into the equipment and that is a big mess!
If the algae is left too long, it can actually penetrate the pool surface and can cause damage over time and becomes part of the pool plaster so it doesn't take as much for it to reoccur.
What to do if you have this pool problem?
Your plaster/pebble surface can be acid washed or chlorine washed (depending on what is needed) to help save the surface. Your filter and all internal filter parts can, and must, be cleaned. That will help the looks and function.
What makes algae get started? Several things, but it is usually a combination of factors.
.....Is your pump and motor running at least10 (ten) hours per day for chemical pool and 12 (twelve) for a salt pool? If the equipment isn't running, the water isn't being filtered and it makes for an easy cesspool effect.
.....Is your gardener dumping grass and leaves into your pool? That really is …