Note that this was a big deal not because 45K was a lot of memory in general by that point, but because the first 640K ("conventional" memory) is special in DOS

Note that this was a big deal not because 45K was a lot of memory in general by that point, but because the first 640K ("conventional" memory) is special in DOS. If you didn't have enough of it free, many programs simply couldn't run, so having DOS itself take up significantly less of this memory was a big win.