It may because there are certain key words that are used as commands, descriptors, and otherwise - in either mysql or php.
And if it is a reserved word of sorts, the typical practice is that using quotes allows you to continue using the reserved word as you desire provided there is an extra quote string.
For example, the mysql documentation on this can be found here: reserved-words.html
You'll see why - in the same reason - if you name your column "desc" for description, when your select statement is made, it will do the same thing making it look like 'desc' - to denote that you are using a reserved word for your own descriptor.
Hope some of that helps,