Bookkeeping is more than just entering receipts. It's tracking every transaction - all the ways that money makes its way into and out of your business. It's a field unto itself. Doing it consistenty, accurately and efficiently, as well as providing reports, and managing the substantiating documents and workflows that feed into the process, requires more than a part-time interest in numbers, software, reporting and procedures.
Because it's not a revenue source, companies often devote partial attention to it. Partial attention does not get this job done. And, every time we do a job half-way, we leave a mess that spreads to other areas.
It's not that bookkeeping can't be done, or done well, in-house, of course, it can. The benefit of outsourcing is that you are starting with someone who's chosen the art of bookkeeping — of making it easier for you to do what you do, by maintaining your records and improving related procedures — leaving you and your people free to work on your business.
 CHECKS AND BALANCES
The outsider point of view is fresh (and inherently critical); it questions everything in order to understand how the business works and what its current bookkeeping practices have been. This questionning naturally sheds light on mistakes, inefficiencies, and fraud. Having to describe how you do a thing usually leads to how to improve it.
Best of intentions and skills, an insider is not as likely to think out of the box or notice assumptions held within a company, that act as obstacles to doing it better, faster, and on time.
Not to say that your people don't do what they say they will, but things come up and interfere with our schedules all the time. Your employees have many responsibilities to the company. As do you. Priorities will always conflict. An outside bookkeeper has only one commitment to your company. And, everything about the work she does is based on the premise that consistency is the foundation.
 NETWORKING OPPORTUNITIES
Because the bookkeeper is working with other companies all the time, companies who may be in a similar industry, of a similar size, or structure, or m.o. — or not, you will benefit from her exposure to other company's successful and unsuccessful practices. As a matter of routine, she'll be steering you toward proven best practices and away from seemingly good ideas (or good enough!) that failed other businesses like yours — or very different from yours (you never know where the next great idea will show up!).
You benefit not only from her experience with other company's bookkeeping practices, you also gain the possibiity of connecting your business with other businesses as vendors, as clients, as marketing partners.
Your bookkeeper is in an unique position to know which relationships might benefit you.