I have always used sword/axe in both hand, since I don't know what's good in dagger? They have really low damage for example I am lvl ~70 and my sword hits 300 and my axe even more but all dagger hit 100-200. It's so bad,why should I use dagger??

Consider your spec, for example Sub would benefit with daggers a lot more. Look at the DPS the daggers provide as well. DPS is the overall damage they will inflict, taking into account damage and speed. Generally, higher DPS is better.

Do some dungeons that are your level, you'll get some nice drops.

Well good sir, your moves will do more percent damage of your weapon if a dagger is equipped, if you read about the moves it tells you