I think Pen vs mouse isn't the issue. To me, the question is: did you do it all yourself, to the last line (including choosing the thickness, the color… of it all), or did something do it on your behalf, based on higher level instructions?

When you draw by hand, you are directly responsible for everything that ends up being on the paper. Nothing ends up there that you did not deliberately put there. So you get to know every what every single line, every single line style is for. You wouldn't put them there otherwise.

When you draw with the computer, you ask it for something, and it produces some output. But what makes computers efficient is that they do a lot of the work for you. So you do not digitally draw every single pixel yourself. You ask for a screw, a window, a light fixture, and you get one. It's much faster (and possibly prettier), but you are not necessarily getting familiar with every single piece of the drawing that gets produced when you ask for one.

If architects (or mechanical engineers, for that matter) don't really need to know what a thick or thin line is for, or what the parts in the drawing of a window or of a ventilation system mean, then they don't need to draw my hand. But if they do, I'd argue that learning drawing by hand does matter. (Or in some pixel art program, but ain't anybody got time for that.) Once you do know it all, use whatever too, but start by learning the basics of your craft, thoroughly.

Automation on top of understanding is great. Automation instead of understanding is fast, until it's a source of mistakes and confusion.