Re: Loading boat on trailer.
back to the OP's question, and leaving aside "using the motor to move up on the trailer":
If you can hand-pull your boat all the way to the winch stand, you are sunk deeper than most people would. If the ramp has any slope to it, you then often have problems keeping the boat straight, especially if there's wind or current. To get that deep, you often have your vehicle in the water and probably have difficulty at the tongue without getting your feet wet. And the more you sink a trailer, the more problems you have down the line with rust and wiring issues, especially in salt water. but if it works for you, fine.
On the other hand, the less you have the trailer in the water, the more you must use the winch and the straighter the boat loads. Your trailer lasts longer. Your feet stay dry. It's simply a balance of ease and effort, and the advantages or disadvantages of each. For every boater, those factors may or may not be important--for example, for those of us who boat when the water is cool, wading is never part of the process when done correctly. Or here, where it is better not to sink a trailer in salt water often, people invest in a power winch for a larger boat; they winch up a smaller one. The boat loads straight, your feet are dry, your trailer lasts, your ramp lasts. Many problems solved with one device.
Ask why are all trailers equipped with winches and why do they have cables that reach all the way to the end of the trailer?