Think of everything at your home that's connected to your WiFi network. The devices can be a laptop or two, tablets, smartphones, streaming sticks and gaming consoles. If you then start connecting smart thermostats, smart lights, smart speakers, etc to your WiFi router, your WiFi can slow to a crawl. Or worse, your WiFi router will begin to kick off devices when it can't accommodate them.
Modern, up-to-date WiFi routers are very important in the smart home because newer equipment is designed to handle a large volume of devices (i.e. lots of Mysas or other smart home devices).
If your current router is a few years old, it may be time for an upgrade. If you are renting equipment from your ISP, contact them to see if your current equipment can handle all the new WiFi devices on your network.