stretching is when the lights are too high and the plant is reaching to get them, i believe it causes them to become weaker then normal. Not sure of any advantages maybe that you can a really tall plant but that isnt always good. if you want to stretch your plant, move the lights higher i guess.
"stretching" is not a good thing; unless your nodes are so close together that you just want to play with it. but in general, stretching causes the plant to be tall and lanky and weak.
there aren't really any advantages to stretching...it's something that most, if not all growers try to avoid...it makes the stems weak and the plants are likely to produce "popcorn" buds. hope this helps!