I'm assuming you know what HTML is, XHTML is very similar.
A) Differences between HTML and XHTML
XHTML vs HTML
The Most Important Differences:
* XHTML elements must be properly nested
* XHTML elements must always be closed
* XHTML elements must be in lowercase
* XHTML documents must have one root element
B) It helps keep your code clean and easily understood. Similarly, if you're reading XHTML written by other people, it should also be more easily understood than messy HTML.
It won't help you design anything, just keep it organised.
C) Following XHTML guidelines will keep your code clean and in theory make it more compatible with the majority of browsers.