Why are carbonates important in sedimentary rocks?

Lots of reasons. Here are a few:

1. They often contain fossil remains which helps determine an age.

2. They indicate environments of deposition ranging from fairly deep ocean, to sabkha, to desert lakes, etc.

3. They represent storages of atmospheric CO2. CO2 dissolves in water forming carbonate ions. Many organisms construct hard parts from the ions, and when their remains get buried, it's like storing away atmospheric CO2.

4. Specific types of carbonate sequences can indicate things like changing sea levels, migrations of reefs, fossil successions through time, etc.

5. Modern reefs provide important habitats and protection from dangerous surf.

6. Carbonates provide useful materials such as lime for cement, building stones and tiles, a Ca boost in animal feeds, chalk, and an additive in toothpaste, paper, and paints.