[danpat]

Depending on how anal you are, and how complex your invoices are, one good reason to use a tool over a Word template is simply adding the numbers up properly.

I joined a small business that was doing them in Word. After reviewing the last 12 month of invoices (about 200), I found about 5% of them had simple math errors, i.e. the Total did not match the sum of the line items.

Sure, you can get it right by hand, but why bother when using software can eliminate the adding-up step entirely?