Yes, you can buy the cellular-capable Apple Watch and just not connect it to a cellular plan. There are two reasons you might do this.
One, if you really aren't sure which to buy and would like to have the cellular option for the future. If this is the case, then don't set up the cellular service plan on purchase. Then you can decide at any time to activate (and pay for) the cellular plan.
The other reason you might make this decision is if you don't care about the cellular plan at all and you just want one of the higher-end watches constructed from more expensive metals and have sapphire faces, i.e. the stainless steel Apple Watch Series 9 or the titanium Apple Watch Ultra 2. They both come the cellular capability whether you want that feature or not. But you don't have to activate it if you don't need it.