What is VBA used for?

A common use of VBA is to add increased 'functionality' or some 'automation' to the various MS OFFICE programs.

For example, you could access a commonly used menu item that you commonly use, such as the 'Statistics' tab from the document 'Properties' item from the 'File' menu, which is not easily accessable, to be instantly accessed by a hot key.
Or, you could automate a series of commonly performed procedures or actions. For example, you might commonly enter data into a table then do some calculations and/or formmating of that data. VBA can be used to automatically do the calculations and/or formatting of this data.
To get some idea of what can be done using VBA, do a web search for 'VBA Addins'.