Simply because it is the best tool around of this kind. It provides almost everything what somebody working with COM needs. It is very easy to use, has many details showed in a friendly environment and user can find what he or she is looking for very fast.

When looking for some ActiveX control/COM object/component fast sorting by specific criteria is crucial. In the same way you want to know what is the functionality of some control, what can you do with it.

The program shows a list of components which provides fast sorting by ProgID, CLSID, Description and Status. In second part of the program there is detailed type information (interfaces, methods and its parameters, properties, variables, etc.). Detailed type information means that you can find out what methods and properties some component provides and what does some method do, or all constants and their default values defined in component, or how much memory it takes, etc.

There has always been the need for getting information about some components functionality. COM Up! offers the overview of all the interfaces, methods, properties and similar. You can see what the component does in a few seconds and you don't need to search its help file or create test applications to see what the can component do.

Web developers can use, in addition to type information, feature of creating test HTML page which containing some ActiveX control. That way you can view ActiveX control in a web page.
When you select some COM object in the list, details and registry entries for that component are automatically showed. Registry entries are especially useful because you see merely all data some COM object has in the registry.

As a matter of registering components, it provides much better solution than regsvr.exe(regsvr32.exe) because it shows error information in case registration fails. It can happen that file isn't COM component, it has no DllRegister function or this function failed to execute, or there simply isn't enough memory or something else.
Unregistering provides user with rich error information in the same way as registering does. Some COM object's registry entries can be removed without actually unregistering its server file and other registered COM objects of that server file.