Simply this benefit is that if the flight you are looking is sold out in the cabin you're looking for, as an Elite Plus, you could pay full fare J or full fare Y and fly on that sold out flight which you must do at latest 72 hours before the flight.

For domestic flights, it's pretty useful as due to the price cap on Economy tickets, an Elite Plus could simply pay TRY 990 for B fare class ( highest domestic only fare class ) and travel on any flight that is sold out and travel as some domestic flights in Türkiye are tend to sold out weeks in advance.