At the beginning of the game, when there were two skills I wanted that each could be leveled up on character level 12 (for example), then I would have to save at least one skill point before that so I would have enough. Keeping skill points in reserve allows you the flexibility to try out different tactics, etc when required, or get a skill that you didn't know you would need, or that would come in handy. As a warrior I bought many of the mage skills I had;
Free skills, and spell books you can buy spoilers
Level 5 of Know Creature shows an NPC's resistances, which is very handy for mages in deciding which spell to use against them.
