It's pretty common practice for manufacturers to upload their custom system apps to the Play Store. This way, the individual apps can be updated outside of a system upgrade. Sony has done this for many of its applications, and the 'Xperia Assist' app is the latest one to be uploaded.
If you're not familiar with it, Xperia Assist is an app designed to guide users through various options and features on select Sony phones. It's included on quite a few devices, including the Xperia Z5, Z3+, XZ, XZ1 Compact, and more.
The changelog simply says, "First release of Xperia Assist," so it doesn't look like there's any new functionality quite yet. You can grab it from the Play Store at the link below, but it will only install on select Sony phones. The app is also available on APKMirror.