Often overlooked, the cables used to connect everything are often very cheap quality. That means usually they're the weakest link in the signal chain. Poor quality connectors and inferior shielding can and will compromise the video and audio. Why buy a great television or top-line receiver if the cabling prevents them from giving you the viewing experience you've paid for?
If you have multiple devices (Freeview receiver, DVD player, VHS, etc.) a SCART switch box will allow you to choose which signal to route to your screen. Some detect which device is active and will switch automatically for you. Others may require pressing a few buttons.