I use an app. It's called mail.app. FedEx, USPS, and UPS email me directly when a package is coming to my house.
Many people find email confusiing and hard to use. No kidding. Likely none of those people are reading this here (on a Mac rumors site) but you'd be surprized.
The better questio to ask is "Why are these so many little apps like this when the same funtion could be done on a web page?" The answer is easy -- No one will pay for web pages, apps exist so the author can collect money.