If you leave them ON the dock, then how do you tie up when you return? You can't, until after someone jumps on the dock, or someone on the dock throws you a line.
Where if you take the lines with you, you can always throw a line around a bollard or horn cleat and tie the boat to the dock, at least initially, without ever getting off the boat.
(Folks who run jump to the dock usually wind up falling on something at some point.)
This depends on the boat. I step down 18 inches from the lowest point of sheer on my 41' ketch, but mine's an older boat. A lot of newer vessels have developed very high freeboards; however, though I often leave lines at the dock, I always have lines aboard. Take care and joy, Aythya crew