There are a few benefits to adding a new rank above immortal.
High ranking players would be able to queue with their friends again. This means less reasons to smurf.
Players who reached immortal and stopped playing on their main account because of fear of losing their immortal status would play on their main accounts to try and reach the new rank. This also decreases reasons to smurf.
Account sellers would have a harder time selling their accounts because it is harder to reach the new higher rank. Many account buyers would only want the top rank.
A new higher rank would help differentiate game information. immortal rank being from 5660 to 12000 is too big of a gap. With a smaller range players could see what actually happens in true high rank games.
Im sure there are other benefits. What are your opinions?
Source: https://www.reddit.com/r/DotA2/comments/xpw9th/do_you_think_they_should_add_a_rank_above_immortal/