Echoing Rampant and bigG, stretching (or, pre-stretching, if you will) the strings, as Rampant describes the process, is essential to properly stringing your guitar. It's not so much that the strings get stretched as it tightens the wraps of the string around the tuning machine's peg....like when you pull a knotted cord tight...to stabilize the bond of string-to-tuner and minimize the possibility of the string loosening its grip, thus lengthening itself and going flat. Loose strings render the best of tuning machines useless. It's not the tuning machine that's defective -- it's Operator Error. :