We created Appstage because we needed a single simple method to deliver apps we've developed to pur clients. As an XP influenced company we also believe in frequent and often delivery of software. To this end we wanted a simple web space we can share with out clients that allows us to produce professional weekly deliveries as well as giving live build access to the client.
You can if your client has given you access! Some companies prefer no to give access to the appstore to external vendors which then causes a problem distributing builds to them. Appstage also allows you to deliver the source code, notes etc all in single place.
We are strong advocates of continuous delivery to allow fast and frequent feedback from the client and to avoid release hell. We've found our clients love the fact they access the current developement progress at any time through the love build page.