What is T-Mobile Diagnostics app used for?

My phone has been sending messages without my knowledge, as well as erasing certain abilities such as being able to pull up my keyboard while messaging someone. It seems like something or someone is messing around with my permissions, so I was looking through the "special permissions apps" and came across "T-Mobile Diagnostics". When I googled it, it said it was a downloaded APK file, but didn't say why or who downloaded it. What is T-Mobile Diagnostics version "80231-808927712-REL-DIAG-2023.01-dark-release" used for?