Why Google Apps? 1. Risk Transfer. There are a lot of risks inherent in managing email: viruses, spam, loss of confidentiality, database integrity, and service uptime ... just to name a few. The risk for managing email is transferred to Google. In theory, Google is better at managing services than you.
2. Disaster Recovery. Along with managing a service like email comes the burden of managing backups for disaster recovery. Again, Google takes care of all of that. Google is better than you at managing data and recovering from loss. If a PC gets lost or destroyed, we just access Apps to get our data back. Nothing's ever lost.
3. Scaled Investment. You buy and pay for what you need under a subscription-based model. Under an ownership model like owning your own server, you purchase an asset that can deliver a maximum capacity, and you pay for that excess capacity both up-front in an acquisition cost and through the life of the asset in maintenance cost. Shouldn't you only pay for what you need?
4. Email Everywhere. Under Google Apps, your inbox (its folders, email, contacts, calendars, tasks - everything you see in Microsoft Outlook) is available to you on every device everywhere. If it's sent from your phone, it's in your Sent Items on your PC; if you filed it under a folder called Rocky, the Rocky folder is available on your tablet computer. There's no distinction for when and where you receive stuff.
5. Ubiquity. Finally, one of the best features that I like about Apps, is that it can be used on any device and in any combination of software. Mac, PC, Linux; Droid or iOS; tablets, laptops, or PC's. If you're a Microsoft Outlook fan or just like the ease of accessing email under a web browser like Chrome, the service accommodates. It's flexible enough to fit with any end-user preference.
These are some strategic reasons why you want Google Apps for your small business. There's plenty of technical reasons but really - this is the kind of capability you want out of an information service. Why mess around with owning capability when you can lease it, and shift the risk to a player like Google who can manage this stuff much better than you?