Protists are ecologically and economically significant organisms; they are responsible for a large portion of the Earth's fixed carbon, serve as important primary produces, and some are important plant and animal diseases, some of which have caused considerable devastation to humans and their crops. Increased nutrient inputs to our lakes and oceans has caused economically important "blooms" of these organisms.
Considering their small size, their diversity is astounding. The beginnings of multicellularity in early protists paved the way for even greater structural complexity and diversity in taxa such as plants, animals, and fungi.