With main-lining, you create a plant where every cola is exactly the same number of "steps" away from the roots. All colas originate from the exact same part of the marijuana plant, which naturally causes each cola to receive an even amount of nutrients and energy.
As a result, plants grow with an even, flat canopy, which produces the best yields for indoor growers using grow lights. Many different training techniques can be used to produce an even canopy, yet main-lining seems to be one of the most effective technique for small-scale marijuana growers.
With main-lining marijuana plants, the purpose is to build a "hub" off of a single node, creating a manifold for equal energy distribution from the root mass to the growing tops.