Dear Big Uncle,
Your question is so basic, but I guess it is necessary because everyone needs to know the role dharma protectors play in our spiritual path - only with this knowledge and clear view, that we could benefit from praying to the protectors.
Actually the role of protectors can be found right in the centre part of this Homepage:
Protector:
Also known as Dharmapalas, are special beings that emanate to protect the authentic Dharma.
They do this by clearing obstacles and providing conducive conditions for Buddhist traditions to propagate and grow.
Some Dharmapalas are Buddhas and bodhisattvas appearing in worldly form, whilst others are spirits who have been subdued by compassionate lamas, and bound by vows to assist practitioners.
As emanations of Manjushri, protectors like Kalarupa and Dorje Shugden belong to the first class of Dharmapalas who are enlightened beings appearing in the guise of worldly protectors. Thus to invoke their blessings, is to invoke the assistance and blessings of an enlightened being.
Hence when we request for assistance, the motivation MUST be pure. For example if we request for wealth - the motivation should be to have our basic needs fulflled AND be able to support the work of spreading dharma and helping others. Only then our wish for wealth will be granted. And if we wanted wealth for our own enjoyment, it would not be fulfilled.