Most of the uses of protists you will find have to do with the algae.
Algae are used for in the environment for:
1. food at the beginning of aquatic food chains
2. camouflage on some organisms
3. habitat for sea urchins, sea otters, ...
4. producing oxygen during photosynthesis
Algae are used by people for:
2. fertilizer on gardens
3. agar for bacterial cultures
4. extracts for making cosmetics, pudding, ice cream, etc. smoother and creamier
5. abrasive substances in polishes and toothpastes