Question

Why is task of compiler construction broken down into phases?

Answer 1

transforming the AST into some compiler specific middle-end internal representations (IR); notice that some compilers accept several languages (e.g. C & C++) by defining a common AST for them (and having several parsers).

for general optimization purposes, repeatedly transform these middle-end IR and simplify them (e.g. dead code elimination, constant folding, loop unrolling)