do you mean like this: (i rushed out a pen sketch...ignore the bloke underneath!)
People do this for a few reasons. 1: letter stucture is v.important... as you get more complex leters its important the letters are still being formed from their principle parts(these principle parts are the blocks), and this technique makes sure u get that.
2) consistency. Letters, especially at first should be generally of the same width for most styles, and this helps with that.
3) erm... just generally if you look at letters they are made up of these blocks... try it... it helps!