How to I obtain a spell doc and use its ability? What purpose do they have in the realm?

Spell documents can be given by specific players who have the ability to reward that particular level of that document. Some spell documents have more or fewer levels, and each level contains a small portion of the whole spell.

Spell documents are related to the inner magic side and are not as easily "castable" in the same way that outer magic spells are. If you want more information on outer magic, the spells you can cast in MD chat, see here.

Spell documents are awarded by players with the authority to grant them.

To obtain a spell document - find a player with such ability, and complete their quest.