The point of quotation marks is to “set off and represent exact language (either spoken or written) that has come from somebody else. The quotation mark is also used to designate speech acts in fiction and sometimes poetry,” according to the Purdue Online Writing Lab.
A secondary use of quotation marks is to indicate irony or when a word is not being used in its current commonly accepted sense. For example, The lunch lady plopped a glob of “food” onto my tray and Crystals somehow “know” which shape to grow into. (Both are from the Wikipedia entry on Quotation Marks.)
Quotation marks should not be used to give emphasis. This entire website is of quotation marks used incorrectly.