Who is your internet service provider?
What kind of connection options do they offer?
For some services, one receives only a modem.
For others ISP's provide a combined "modem/router" -- often called a "residential gateway".
Again, what you use depends on what kind of connection you have to the ISP.
There's nothing wrong with using both a modem and a router.
It "is what it is".
I have a residential gateway from my ISP, and I've disabled wifi on the gateway and use my own router (Linksys Velop mesh system).