Chromecast connects to any hdmi tv and can stream any compatible streaming app from your phone/tablet/pc over wifi. One of its purpose is to make a dumb tv into a smart tv but it can do more than that for convenience reasons like if your tv is limited to how many apps can be installed, or have garage tv or you simply want to use the device's keyboard to search videos instead of using the remote.
I have 70" 4k tv and 1080p content looks fine. Most 4k tvs upscale.