For three main reasons:

1. To provide complete and updated stats and charts, it needs to download your orders and sales data everyday from your store and email accounts. If Mopapp was not a web-based service, you'd need to remember to run it everyday...or at least use the computer everyday so that it could automatically run. And this might be a problem for most users.
2. You won't need to worry about power outages, system failures, slow performances, data security and data backup. We'll take care of everything.
3. We'll keep improving and evolving Mopapp, and we already have a long list of new features we want to add. Being a web-based service, we can update the software on our servers, and all users will get the new functions instantaneously, without doing anything to upgrade. Ah, the pleasures of the web!