You can definitely line breed, in fact you can end up selectively breeding to produce lines of snakes, displaying specific traits somewhat unique to the line. Often a few generations of outbreeding will get rid of these traits.

Line breeding can also create morphs by isolating certain genes in the lineage, when a mutation occurs genetically you can seemingly have a spontaneous morph appear and can be line bred to prove out the morph. There are a lot of positives to line breeding.

Sometimes, however, you may be perpetuating unhealthy traits, such as neurological disorders. It is important to selectively breed for this reason, removing genetics which are causing issues, and continuing desirable ones.

In other words, breeding is eugenics for kids!

All that said, Lizardlicks is correct, you want to outbreed from time to time.