This is quite a self explanatory idea (a statement to run whilst the condition stated is true) as is a fairly basic command in most languages and i feel is missing in PowerApps.
The ForAll statement can replace a while in SOME situations but not all. For example, if you want to check if a table has been refreshed - a ForAll loop doesn't really help you!
A while loop would greatly improve efficiency for me instead of having to use longwinded workarounds which waste both a lot of time on myside writing it, and on the users side as it is resource heavy. Please add this feature as it is a common sense command that would aid a lot of people!
Bensack on: ALM for PowerApps DataFlows
Upgrading to under review and adding @GregLi to take a look.