Carrier Services is the application developed by Google so that mobile devices running on Android and with a connection to telephone data networks can make the most of the latter. It's a component that keeps up to date the all the specifications of the so-called IMS architecture that provides support to data, voice, and multimedia traffic.
Your Android device with the latest mobile connectivity specifications
That means that to make a device compatible with the latest network technologies, it won't be the manufacturers or carriers who have to update: the process will be much more simple thanks to the fact that it will be Google who will do so through Google Play, just as if it were an ordinary application, reaching more and more devices. However, that does hinder those users that use Android without making use of Google, although they're a minority.
In turn, this Carrier Services app includes support to improve the features of Android Messages. That's the operating system's own messaging application, similar to Apple's iMessage for iOS, that resorts to the RCS communication standard. The latter allows us, for instance, to send images or group chats without needing to have an Internet connection. However, that doesn't mean that if you've got both apps installed that you're going to able to make use of these functions: not until our carrier accepts.
Initially, Google released the application's APK only for Pixel and Nexus devices updated to Android Nougat but it has gradually landed on the rest of devices. Therefore, if you want your Android to be using the latest connectivity standards as soon as possible, you had better download this app right now.