Like other online applications, it has inherent advantages over traditional software. Collaboration is easier, for one. Although Gliffy doesn't offer real-time group editing as Writely, Google Spreadsheets, and SynchroEdit do, it makes it easy for multiple users to work on a diagram in turns. Gliffy does track all changes made to a document. It's kind of like a wiki in that way.
Gliffy also makes it easy to publish a diagram you create to the Web. Gliffy keeps a live JPEG of each file on its servers, so if a site or a blog links to that image, it will always show the most current version.
Gliffy is still early beta. While I found the application stable, it doesn't have a feature set that'd make it competitive with Visio. I found it hard to align objects, for example. Also, the library of objects (flowchart shapes, networking equipment, furniture items, and so on) is limited, and users can't add their own items. And there are no dimensioning lines, so it can't make accurate floor plans.
But this is a very impressive start. Gliffy has enough functionality today to serve as a good sketching tool for creating simple flowcharts, network diagrams, and user interface mockups. And you can't beat the price.