Is there a purpose for the Magnificent bird of paradises tail feathers?

It is to impress the females.

Generally, male birds have some way of 'showing off' to potential female mates and he will use this to attract the ladies.

The females will be attracted to the male who puts on the best display.

Display methods vary from species to species - some simply have a different coloured beak, some will dance, some will make impressive sounds and sing songs, some have bright coloured feathers and some have REALLY fancy feathers which can be used to impress the ladies.