T-Mobile tends to (usually) compete on price with small freebies thrown in, but now it's sweetening the pot with a bonus you're likely to use. If you have a T-Mobile One family plan (that is, two or more lines), you now get a standard two-stream Netflix subscription for free. It'll cover both new and existing Netflix viewers, and you can apply the credit toward a more advanced Netflix tier if everyone in your clan wants to watch at the same time.
There are a few catches. You need to be on a One plan, of course, so your grandfathered plan won't cut it. Also, Netflix streaming on cellular still defaults to 480p -- you need to enable higher quality yourself. And there's a degree of lock-in involved. After all, if you switch carriers you're suddenly back to handling the Netflix bill yourself.
The network isn't shy about the reason for the move: this is a foil to AT&T's DirecTV Now and Verizon's Go90. T-Mobile claims it doesn't have an interest in snapping up a TV provider or internet service to offer video to its customers, and that many people would just want Netflix anyway. There's a degree of truth to that (there's a good chance you already have Netflix), but it's clear that T-Mobile's deal was born out of competitive pressure.