Why I use Sans-serif fonts like Arial in my PPT files.
A lot of people use fonts like times new roman in their presentations. Times and similar fonts are great for blocks of text on a book page, but there are issues in presentations where you have small amounts of text and want maximum legibility. The Serif fonts tend to get hard to see at small sizes reducing legibility. Here is an example to show how Arial, for example, is easier to read than Times New Roman, especially at smaller font sizes (or large font sizes viewed from the back of a large lecture theatre).