Why do we apply the concept of circuit in homomorphic encryption schemes? I am a beginner to the concept of homomorphic encryption. One thing that makes me very confused is why we use the circuit concept or approach to homomorphic encryption. Gentry's paper does not discuss it in detail. Are there any papers or books could make me have a better understanding of it?

(I mean the circuits part.)
We use circuits because they are universal (any function you want to compute can be expressed as a circuit) and because they are convenient (because we know how to solve the problem of fully homomorphic encryption for circuits but not for other models).  In particular, circuits are in some sense simple: all you need to do is find a way to implement an AND gate and a NOT gate, and that's enough to be universal.  This helps avoid inessential detail and helps us focus on the core of the problem.