Yes, it is a pain to level, about the same as Runespan and RC. In terms of uses, there are many.
First,
divine locations. Great for training skills and a way to make money, too. There are Hunter, Herblore (collection grants Farming Exp), Woodcutting, Fishing, and Mining nodes. Players can make these once daily and collect up to a certain limit.
Second,
signs. These activate when certain conditions are met. There are four of these: respite, porter, item protection, and life. Respite adds more time to your gravestone. Item protection is self explanatory as is Life which is essentially a ring of life. Porters are particularly the most useful. Porters teleport collected items to the bank. Again, this is helpful with skilling and making money.
Lastly, you have
portents. These are like signs, except they are activated when falling below a certain amount of health. These mainly deal with healing the player. It is worth noting the portent of passage allows you to pass through doors you normally couldn't open while Dungeoneering.
I'd check out the whole wiki page on
Divination if you want more info, but that's a quick summary of helpful things the skill can do for you once leveled. Since you are maxed, idk how much good it would do you now.
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