A: It depends on the size and the nature of your business. Bill.com has a lot to offer businesses with a lot of...well...bills to pay (aka payables or accounts payable). It also offers invoicing (aka receivables or accounts receivable). Bill.com syncs up with your quickbooks accounts online so that your bookkeeping is easier. Sure, you can use quickbooks online for these same things, but Intuit's interface and user experience is cumbersome. Bill dot com makes it easy to delegate responsibility to an administrator without risking that same person messing up your books and billing and risking upsetting your accountant and/or CPA.
The way that Bill.com provides value is by saving you time and organizing your cash flow so that you can manage the timing of what goes out and see what is coming in. It can give you a feel for "what is yours" and what is on its way out the door and help you become more of a paperless operation. (It has a mobile app for iphone) It is not used for general expense tracking or bookkeeping (like Tallie or Wave) but it does act like Freshbooks for invoicing so if you are already married to Freshbooks (she is pretty hot) than the value of Bill.com is less.
Here is an example of how the invoicing and payables could help you. You have a startup business in your basement in Northampton, MA. You have a lot of businesses sending you bills for the products that you are reselling on your ebay and etsy store. You find yourself spending a lot of time just trying to keep track of who you owe how much and if you can afford to pay your helper / contractor at the end of the week. All you want to do is go to Asia and buy the items. With Bill.com you can have every bill scanned to a special inbox and then all you have to is have your helper route them to you for approval. Clik clik click, looks good, oops that looks like we should dispute it, etc.