NII bidders apply on the last day of IPO mainly on account of two reasons:
NII bidders generally use IPO Fundingto invest in IPO. Thus, applying on the last day of the issue helps them save on the interest cost.
Secondly, NII investors cannot withdraw or lower the size of their bid in terms of quantity of Equity Shares or the Bid Amount at any stage. Thus, they apply at the end when they have a fair idea of the IPO oversubscription figures.