Q. Why confess to God when He already knows what sins we have committed?
A. It is not that God needs to be told but that we need to admit what we have done.
The sages say that the kernel of the confession is the words, Aval anachnu chatanu – “Indeed, we have sinned”. Unless we own up, there is nothing that we (or God) can do about it.
Maimonides begins his Hil’chot T’shuvah (Laws of Penitence) by stressing that the first step in spiritual rehabilitation is viddui – confession.