It's to stop your face from getting pounded! As a beginner, it's best to concentrate on having a high guard so if you don't move/slip etc (which is quite likely as takes a bit of practise) the punch coming to you will be at least partially blocked. If you try and have a low guard like floyd starting out, more experienced fighters will be getting in clean shots all day. Also floyd alternates his guard. He usually has his right hand up by his chin stopping opponents shots with their left and has the right across the body to stop anything from landing there with the chin tucked down into his left shoulder, using his reflexes to get away from the longer (therefore slower) right hand punches (obviously this theory only applies to orthodox). He does use the high guard as well, usually a the beginning of a fight, probably to keep it tight first off and not being caught cold. Also Floyd has been boxing since he could walk, so can get away with alot of things no one else could I think its an experience thing. With more sparring you know what you can get away with and what things will end you up in trouble, but for the most part it is better to have a high guard starting out.
My advice, just get in the ring and see how it goes. It's only sparring, you spar to learn not to win. Generally i just hit as hard as my spar partner, it often seems to get harder during the bout, nut it's all good fun and it's in a controlled environment. And in the amateur game it's speed you want to concentrate on, power is good yes, but think about it like this. If i land 10 light, fast but soft shots, i have 10 points, if you land 9 bombs but fail to take me out, i win. It's easier to land and score with fast shots than it is with heavy shots