HTC like some other OEMs such as Samsung and Motorola have been dumping a number of their brand specific applications onto the Play Store for a little while now, which enables users to have an easier time getting updates for those apps without having to wait for larger, more drawn out software updates to the device itself. HTC wants to make signing into those various different apps easier and have now released an app to the Play Store for HTC device owners which can help with the process called HTC Service-HTC Account. The app's goal is fairly simple and only applies to HTC device owners as no one should expect that an HTC app (especially one such as this) would have support for non-HTC devices.
As HTC devices also come pre-loaded with a handful of different apps already on the device similar to the way other OEMs pre-load their own content, HTC Service is a way for users to easily sign in and integrate those apps with their HTC account. Using the HTC account was something users had the capability to do prior to today, but now just like with other apps HTC has put on the Play Store, the company can more easily keep the HTC Account features up to date by simply updating the HTC Service app.
This streamlines the whole process for them if they ever want to add in new functionality to the HTC Account for users, and then users can then promptly download and install the latest update from the Play Store via their device or through the Google Play web portal. If you don't have an HTC device this app unfortunately does nothing for you. Furthermore, the app itself even for HTC users only serves as a means to get updates quicker since the Account app was available already. The app should already be installed on most current HTC smartphones like the HTC One M9, HTC One M8, and so on. With HTC service now up on the Play Store, HTC is continuing the trend of placing any, and likely all eventually, of their preloaded device apps in the store for easy updates in the future.