It is used to hide known file extensions by default. In other words, rather than displaying a full file name like "pcworld.docx", Windows will only display "pcworld"
It is the best way to protect portable systems such as laptops from loss of data and information when the laptops themselves are lost or stolen.
The BitLocker on desktop systems is a good consideration when you consider how much information can be lost from recycled desktop systems that have not undergone a proper hard drive wipe routine before being sold off.
Both b and c.
Both a and c.