I had an XPS that had both and indeed it was useless to have a port for just charging. But if the charging port is also used for magsafe that makes it worth it.
The XPS 15 has a dedicated charging port that can draw much more power for the discrete GPU - up to 130W, but lets you charge it on the go with the USB-C port. You get full graphics performance with one but have the choice not to lug around a larger power brick on the go.
Why would I want only one option when I can have both?