The Future Growth track is meant mostly as a convenience mechanic.
Some events make you Grow Up all in one go, resetting the Growth track, while you still might accrue other Growth marks... so the Future Growth track is a place to park your full "level up" while leaving the normal track available to record more stuff.
Also, you might Grow Up multiple times in one go... the Future Growth is again a place to record the fact that you have 1 or 2 or more advancements to spend.
Finally, in most games I had the Players would get 5+ Growth Marks but be unwilling to spend them immediately, wanting instead to think it over in the time before the next session. Again, they simply parked these full advancements into the Future Growth track.
I believe this is all explained in the current wiki though. Let me check :)
HERE
Since you are allowed to acquire a new Growth option only once per session, you might end up "saving" excess improvements in the form of Future Growth marks that you can then spend during future End of Session moves.