The first is that you are able to get a great deal of storage space, much more than you may be able to get if you were using a client program for your emails. Currently, Duck Gmail for business offers 25Gb of storage space which means that it is possible to store a large number of email messages or messages containing large files rather than being limited by storage space on a corporate server or hard drive.
One that comes instantly to mind is the fact that it can be accessed anywhere where there is internet access. On a server-based email client program you may find that it becomes difficult to access email. Some people find that when they access an online version of their messages they lack the ability to work with them as easily as if they had accessed them from their regular computer.
Duck Gmail for business also integrates features such as IM storage and video chatting which makes it easy to get in touch with clients and co-workers and save the results of those conversations for future use.
There are also a number of different features that allow you to organize your emails. One exciting feature allows you to add Hotmail and Yahoo accounts to your Duck Gmail account so that you are able to manage all of your emails from one account.