You have a later part in a song or score you want to stay where it is always - in time - while adjusting earlier in project.
Obvious when you work with video scenes.
Suggested solution #1 - MIDI regions
If you create a midi region it is anchored in time.
Earlier midi than the a region fill up with the tempo is has to that point.
Any number of midi regions are allowed.
Suggested solution #2 - anchor MIDI parts in time, without touching track timebase
This would solve the same thing, choose in midi part basis if anchored bars+beats or in time linear.
What it solves:
Adjusting tempo in one region does not affect postion of midi in another region.
For audio you just have timebase linear that solve this - but that does not help midi parts which are probably larger part of all projects now a days.
When doing video in particular you feel the need to adjust tempo a bit in a scene without changing position of all midi comíng after that.