if you have a Garmin which has the maps in it essentially offline then why do you need to transfer that to a computer?
A Garmin has maybe a 4 inch screen to a max of a 7 inch screen - my basic computer has a 15 inch screen - up to a 34 inch monitor TV we can connect to our computer. I can either see things in a larger version or have a larger playing field showing a lot more stuff. I am old and can't see a tiny little screen such as a phone these days. Very limited amount of information can be seen on a small screen. I can sit at a computer and have several screens open at once and switch back and forth from screen to screen. I can plan a route and save it for future use. I can create a route and then share it online with others as I have tried to do on this forum many times. There are countless reasons to be able to map in a computer - a longer list than I can show here. It is all a matter of whether you like to know where you are going and how the best way to get there might be - and what is along the way - clear and concise details. Every new program could have some new bells and whistles that can add to the experience of figuring out where we are going.
As Yogi would say . . . If you don't know where you are going, you might wind up someplace else.
Yogi Berra