A MacGuffin was originally a film concept, defined as "an element that drives the plot of a work of fiction." Basically, they are cheap ploys that make a hero want to do something, and they've never enjoyed more prevalence than in the video game industry. Why do you want that princess, star, or weapon you can't even use? Because you do, you benighted controller monkey! Here's our tribute to the MacGuffins that deserve recognition for being awesome, terrible, ironic, or just confusing.
The Triforce was left behind by three goddesses (supposedly) and has the power to grant wishes, but only to a person with equal measures of courage, power, and wisdom. This seems to be one of those built-in after school special messages, as someone who already has all three qualities wouldn't have any trouble getting anything they wanted. Or even if you just had power, that's probably enough to put you on the road to success sans a set of wish-granting golden triangles. Of course, if you have no wisdom, power, or courage, presumably that's still equal measures, so we can only guess that Link is forever on a quest to prevent stupid, cowardly, unimportant people from touching the Triforce.
What would have happened without it: Ganon would have been forced to ruin lives by some other, less elegant method, like forming a standing army of desert people who are not so afraid of Master Swords, pretty ponies, and wind instruments. This situation would be moderately less difficult to navigate than the Water Temple.