Like Deb said Line breeding can be done to strengthen a specific trait you like in a gene or even to prove out certain things. I for one don't see a point to line breed beyond 1 generation it can be done multiple years in a row but no need to breed second generation offspring to original parents. I plan to do some line breeding with a few project I have. But its only to produce homozygous combos of a mutation like killer blasts or killer bees or killer spinners