Put simply Chromecast lets you solve the problem of, “I want this thing on my device to show up on my TV.” There are a few complexities and limitations but that’s basically it, you see a thing on your phone/tablet/browser and you want to put it on your TV with one or two taps.
What makes it even more attractive is that it has a list price of $35 and can often be found for less than that. Currently running $29.99 on Amazon.