IDR Plan benefit?

Outside of difficulty in making payments, why would someone apply for IDR? I just don't get it. I've been employed for well over 10 years in a federal position making payments the whole time. Despite this, Mohela is only listing 49 qualifying payments on my direct loans instead of >120. Will an IDR application change this? Per studentaid.gov: "Your Direct Loans listed below are not on an Income-Driven Repayment (IDR) Plan. To benefit under PSLF, you must be on a qualifying repayment plan, which includes all IDR plans." What does that mean "to benefit under PSLF?"