@chowkwan - At the risk of over-simplifying, a router provides a transition between two separate networks, whereas a switch just allows multiple devices to communicate on a given network. Technically, if you only had one device - say, a streamer - you could just use a pure router to get your music from the internet.
Most people are interested in audiophile switches because many on the market are reputed to reduce or eliminate network noise (which is a real thing - networks are noisy places). You might just want to put the switch between your streamer and your router to achieve this. But most people have more than one device, and therefore need a switch. As a side note, many Internet routers have built-in switches so that multiple network cables can be connected. These are of the noisy variety, however, and so an ‘audiophile’ switch may be needed for the best fidelity anyway.
Hope this helps…