x2 what @EggSighted4Life said
Will just add that inbreeding and line breeding are more likely to increase the chances of good characteristics you want to keep being passed to the next generation. On the flip side, it is also more likely to increase the chances of bad characteristics popping up. This is because there is an increased chance of the offspring inheriting the gene(s) required to express the characteristic.
There are also certain traits that remain hidden in recessive genes. When you breed related birds the offspring can inherit the recessive gene from both parents, then the trait will be expressed in the offspring. This can be either good or bad, depending on the trait and your goals.