Chromecast, as most owners know, needs power. It has a micro USB port that can be connected to the power source. Many TVs have a USB port these days. Most Chromecast articles or reviews use the USB port on the TV to power the Chromecast. The Google Support - Plug in Chromecast page also indicates the same:

Chromecast connection

This uses fewer wires - and power outlets - but with this setup, I ran into the following problem. Chromecast is powered OFF when the TV is turned OFF. Now if you use your mobile device to cast content on the Chromecast, it will not show up in your list of Chromecasts. You need to first turn the TV ON, then wait for the Chromecast to appear in the app and then cast content.

I prefer that the Chromecast is powered by an external power adapter, so the Chromecast is always ON. Now even if the TV is turned OFF, the Chromecast appears on the list of Chromecasts you can cast content to. You cast content on to the Chromecast. If the TV is OFF, Chromecast turns it ON, switches to the right input and plays the content. This, to me, is far more convenient.

Not sure if it is just my TV that turns OFF power to USB when it is turned OFF. Do all TVs do that? Are there USB ports on TVs that remain powered ON when the TV is OFF?