If you have 200 amp service in your house, the wiring to your house is at least sufficient for household electrical needs for the foreseeable future. However you may want more discrete circuits in your house than your panel currently holds or you might want to wire in a bunch of new circuits to a location far away from your electrical box, and then it is often worth your while to put in a sub-panel. Presumably if you have 200 amp service you likely have a decent sized electrical box. And even once all of the slots are full, you can get away with a few breaker slots being doubled up. But eventually there comes a point where the box is too crowded to safely work in it.

For instance when I moved into my new house, I wanted to set up a small wood and metal workshop in my garage. My garage is on the opposite side of my house from my main electrical panel so it was worth it to me to hire an electrician to install a subpanel in my garage so that I'm only running one large electrical circuit all the way across the house and through a foundation wall rather than a dozen smaller ones.

Similarly when I moved in, I had 100 amp service to my house and the main panel was both small and completely full. 100 amps would cover my immediate needs but I did want more circuits in the main house. So I could have gotten away with installing yet another sub-panel near the main panel. But I had no headroom for doing things that I anticipate will likely happen in the future (switching to a heat pump for heating and hot water, I can't imagine that my next car won't be electric, and likely other unknown uses). Thus I opted to just increase my service line to 200A and replace the original small panel (that likely was installed when the house was built in the 1940s) with a much larger panel.