For example, they were the first as far as I can tell to launch extensive WiFi calling and WiFi SMS, even extending to some American airlines (at least for WiFi SMS - calling is possible but socially unaccepted on planes).
Wireless carriers as a general rule are all looking at WiFi offload capabilities especially as 802.11ad and up provide higher transmission rates (peak and average theoreticals).
There is a race to perfect and bring to market seamless WiFi-base station handoffs (both ways) in real-world conditions. I suspect that will require a combination of channel (i.e. mobile devices) + wireless network upgrades, so in the meantime carriers need to bridge the gap. This is arguably a pretty good strategy assuming T-Mobile is expecting WiFi to handle even more of their traffic overtime.