Why are estuaries such rich habitats for oraganisms?

They're the mouth of rivers,

where the water can be either salt or fresh depending on the tides.

Lots of things (plants and animal) live there

and the ancestors of ALL land animal lived there about a half billion years ago.

Note that there are animals and plants that live in the sea that never had anything to do with estuaries, and couldn't care less for their existence.

But many of the animals that you are familiar with (like birds) use estuaries as breeding grounds.