In my company we often have to use either 20% or 22% mac to get out.
We have figures published for both of these for the different flap settings on all our limiting rnw's. 737-800/900 by the way. Standard loading nearly always has us around 22-23% mac. Makes all the diff some days.
MTOW is a figure calculated to ensure the aircraft can safely operate out of a given rnw on a given day with give conditions ie wet/dry and temp. Normally it is calculated for the worst case situation ie. the actual CofG being on the forward limit.
Now if you have your load sheet and on that day it tells you that you have an actual CofG on that day of say 24%mac you now know that you are safe useing figures for MTOW calculated for 24%mac. Airlines are unlikely to have figures derived and tabluated for you for every given CofG possiton. For this reason in my company we have tables/(BLT) giveing you MTOW figures for 20%mac and 22%mac.
If you know you have a CofG aft of the CofG on the tables you know you are safe.
In simple it is a method of degrading the safety margin in order to operate the aircraft from limiting airports safely.------
Granted not a great statement the same safety margin exists just the amount the actual aircraft performance on the day exceeds the required performance is reduced.