When billing costs to customers

We just turned on the Bill Costs to Customers option, which then adds a new field to the Accounts page when the account is COGS or any expense account.  This field is "Track Billable Expenses In" and is a dropdown field that let's you pick an account (any account). 

I'm not an accountant, so this is not making much sense to me.  For example, if I'm setting up a new account and it's an expense account (Type field = Expense), why would I want to select a different account in the "Track Billable Expenses In"  field??

The field help says "Help is not yet available on this topic" and the only info I could find in the online help refers to deferring revenue (if you're deferring revenue - which we're not - you're supposed to select an income account here).  Is that the ONLY time you're supposed to use it?