For example, the parallel command - why does it require a distance? This seems to be "overconstraining" the position of the part, since the two edges are parallel no matter what distance they are from each other.
I feel like I'm missing something critical that would clear this up for me but I've looked around and haven't found an answer so far.
You don't have to use a distance if you don't want to - by default, the offset type is set to fixed so you must specify a distance. This gives you more control if you need to maintain a certain distance between the axes. If you just want the axes parallel yet free to move any distance, you would need to switch the offset type to floating.