Do you need a router for WiFi?

I'm curious, could you elaborate on whether a router is absolutely necessary for setting up WiFi in a home or office? I understand that routers are commonly used to distribute internet signals wirelessly, but are there any alternative options or scenarios where a router might not be required? Additionally, how does the router play a role in ensuring a secure and reliable WiFi connection? I'm looking to better understand the basics of setting up a WiFi network and the essential components involved.