A switcher is the cheapest and most traditional way to send sources to one or more screens. Multiple sources can reach multiple screens but with the limitation that the switching is one to one, that is, only one source to each screen. With a switcher you cannot splice sources, or do a PiP (Picture in Picture), nor can you send more than one source per screen at a time.
A switcher is the way to save costs when you cannot invest in a Video Processor, which has more flexibility and versatility in its image handling.