Many people have additional commas in the file that will throw off importing when using just a comma delimited file.

For example, if you have an address formatted as follows:

7 North Pinckney Street, Suite 240

or a number in the following format:

1,234,567.00

The extra comma(s) will throw off the importing treating

'7 North Pinckney Street' as one field and 'Suite 240' as another.  By encapsulating the entire field with quotes,

"7 North Pinckney Street, Suite 240"

The software knows that all the text within the quotes is that field.