Intelligence

The intelligence parameter is used as an indication for the intelligence of a person (surprise!). Intelligence gives you more knowledge, which results in more conversation options and thus different solutions (read more experience Points) to quests. And it gives the mage more possible spells.

Who should use it ?

Everyone benefits from intelligence. It gives you more dialog options, which makes it sometimes possible to solve a quest without fighting, which in turn results in general in more experience points. It also gives more quests that can be done, which in turn results in more experience points. So basically more intelligence means more gameplay!
For the nameless One, intelligence also means that memories are regained faster.
As a mage a higher intelligence increases the maximum number of allowed spells that can be cast. So the higher the intelligence the more spells you can cast.

The Nameless One

At the end of the page the table is given that indicates what the effect is of increasing the intelligence parameter. From this table you can see that adjustment to the number of spells per level start at 9 and the knowledge bonus at 15.

The Nameless One starts out with a value of 9 for the intelligence parameter. Independent of the class I think that you should increase this value to at least 13. You still have the possibility to increase this even further in the game. If you are a mage than set it to 15.

The Tables

The following table lists the modifications that are applied to one's abilities given a certain intelligence.