Update: as of yesterday, some reps seem to be experiencing an issue where when they apply this discount to a customer, the customer's data shuts off completely in which case the customer would have to contact T-Mobile and get them to put the data back on.
Either way, for people that will be on wifi most of the time and will be able to maintain within the 3gb (or 4,5,6 gb if you purchased more), this can save you.
While you can purchase up to 3gb additional for some lines, and keep other lines at the allotted 3gb data, you can't though use this discount for some lines, and keep other lines with unlimited data etc. All lines must have this 3gb capped data applied, and then you can purchase the additional up to 3gb data individually.