Completing IGETC means that you will be considered to have completed general education requirements for most majors at most UCs and CSUs. It is not required for transfer, but it can be nice in that you won’t have to worry about general education courses after transferring to a campus and major that accepts IGETC in place of its own general education requirements.
Even if you do not complete IGETC, you may need to complete some general education requirements before transfer, as specified by the campus (commonly English composition requirements and the like).