Question

What makes functional groups so special relative to other groups of atoms, such as CH3?

Answer 1

Because they are sites of differing electronegativity along a hydrocarbyl chain...........

Explanation:

.........that can be chemically manipulated. Carbon chains are fairly unreactive species. Certainly you can burn them, and we do burn them to provide energy to drive our cars, and power our industries. However, in order to do chemistry on the chain, different functionality must be selectively included somewhere, so that it can be chemically manipulated.

And see this old answer.