> Excuse my ignorance but why would you need this?

There are a couple of reasons why I want to provide this:

Saving your choice of units in the file can end up being a useful bit of documentation that makes it easy for others to understand later on what you were considering "1,0" to be.

If you're working with multiple files from different sources and combining things together, having the unit systems set in each one will allow them to be imported into the current model at the proper scale.

And finally, a lot of people are just really accustomed to setting the units. A lot of CAD software makes a big deal about choosing your unit system as one of the first things you do when you create a file. So a lot of people just aren't comfortable or accustomed to the whole "units are what you think they are" method...

- Michael