I actually make quite a few purchases from amazon on my phone. It's pretty easy for me, especially because I have a lot of items I want to buy vetted out, and in my wish list already. Also, there are times when I'm on the move and dont have internet besides my phone, or spur of the moment purchases I'll make as I think of it, that I use my phone for.

Edit: "moving at the speed of business" is actually kinda applicable here, think it, get it done, move on, kinda mentality when I use my phone. On a laptop, I have a tendency to dilly-dally around my purchases and over-think/read too much into reviews and what have you.