The reasoning is well known, no need to belabor it.
Tips
usage
Almost always appears in fixed patterns: 不必赘言 / 无需赘言 ('no need to belabor') and 不再赘言 ('won't dwell further'). It's a polite way to wrap up an argument.
memory
赘 originally means 'a wart / something extra and useless', so 赘言 is literally 'wart-words', an apt mnemonic for redundant talk.
