NACH enables us to re-invest the money (after it is redeemed into your bank account) into the recommended Mutual Funds automatically. Without NACH, we wouldn't be able to debit your account to do that.
You can refer to this FAQ to understand how to apply for a NACH (auto-debit)
You can refer to this FAQ to understand how to increase the limit of your NACH.
We also have YouTube videos available for all the common FAQs on SIP and Portfolio Rebalancing. This will hopefully make it simple to understand the entire process and help clarify any doubts that you may have.