Freshbooks does give me some features that Excel files wouldn't.

-I don't want to organize a bunch of Excel files
-mobile app lets me send an invoice on-site
-online payments
-client history
-the ability to see if a client has viewed my invoice yet
-send payment receipts with a couple button clicks

There are others, but basically the idea of sending out a bunch of Excel files sounds like a nightmare to me. I have a large number of small amounts that I invoice for, so I'd be sending out dozens of Excel files every month.
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Matt