Simple PHP Theory Questions

[myplacidcasual]

Excuse me for my ignorance.

Why exactly does vbulletin use all these variables (i.e. $vbphrase[ blah] to give you can example)?

Despite my ignorance I can imagine two reasons:

1. So this variable can be changed one time, in one place, and the whole site is updated.

2. To protect information.

My questions are related to those two reasons respectively:

1. Where do I go to find where these variables are defined so that I can redefine them (is it in the actual php file on the server, or in one of the templates)

2. Is there any danger to not using these values. For example I've been using normal html links to image files because its easier for me (a non programmer)

Thank you

P.S. if there are other reasons, or anyone can suggest good resources for understanding (for non-programmers)