Over time a 70/30 portfolio stops being one: if equities run up, it becomes 80/20 without you deciding anything. Rebalancing means selling the part that has overshot and buying the part that has fallen behind, until the weights you chose are restored.
Its main function is risk control, not return: it stops a portfolio from turning more aggressive right after a good run, which is exactly when doing so feels most like a good idea and is least likely to be one.
It is also the most psychologically uncomfortable operation in investing, because it forces you to sell what is working and buy what is not. That is why it works better as a mechanical rule (once a year, or whenever a position drifts more than five points) than as a call you make in the moment.