From D&D Wiki
Bloodlines can give you a free feat, such as Dodge or Power Attack. A character of level 3 or less can take a Major Bloodline, however will be penalized after level 3. Likewise, a level 6 can take an Intermediate bloodline freely, and a level 12 can take a Minor bloodline freely. This can be useful if your character will not level above one of these levels. If you get to level 3, 6, or 12 with a major, intermediate, or minor bloodline, respectively, you are required to take a level of bloodline (which counts as all of your class levels for caster level, so a cleric5/wizard9/demon1 has a cleric CL of 6 and wizard CL of 10). If you do not take this bloodline level, you will not progress in your bloodline and take a -20% exp penalty. This can be useful for characters that know they will not exceed a certain level.