An array formula will return values that would not normally be returned in a single cell like series of cells being calculated against other series of cells. If not entered with Ctrl + Shift + Enter the formula would not be an array formula an error or an inappropriate value that would be returned.

NOTE:

Don't quote whole posts -- it's just clutter. If you are responding to a post out of sequence, limit quoted content to a few relevant lines that makes clear to whom and what you are responding.