Simply that your Chromecast can turn on your TV and set the correct input without you having to turn tv on and set input.
So, how do I do that? First your tv have to support it, and if it’s not too old it will have it (my philips from 2010 got it). Most tv’s today got HDMI-CEC.
It can be called different things by different makers. Like: Anynet+ (Samsung), Aquos Link (Sharp), BRAVIA Link and BRAVIA Sync (Sony), HDMI-CEC (Hitachi), CE-Link and Regza Link (Toshiba), SimpLink (LG), HDAVI Control, EZ-Sync, VIERA Link (Panasonic), EasyLink (Philips), and NetCommand for HDMI (Mitsubishi).
Secondly your Chromecast need to be powered by the external power source, and not from the tv since most tv’s turn off the power in the usb port when the tv is off. So, external power source.
And? That’s it. You don’t have to do more. With the Chromecast being powered up you just have to start casting to it and by doing that the tv will power on and switch to the correct input. No remote need. You have to set it to back to tv mode when you’re done though.