The measure selector allows you to replace one measure with another in all objects at once.
This is useful is you are a developer and has created a dashboard for Sales and wants to create a similar - just on profit.
With TARGIT layouts you can actually do some clever versions of the same dashboard like that very fast and efficient.
It is also a great feature for consumers, who can toggle between measures (if the dashboard is designed for it) and do a lot of different analysis based on just one dashboard.
This tutorial show 3 different clever tips on how to use the measures selector.
(Notice tip no 3 has a call back to the tip from 15th of December - combining user dimensions with measure selector)