Resisting Spells reduces what hostile magic does to you — the duration of effects that lock you down, and the severity of the ones that strip your mana or your stats. It is the difference between a paralyse that ends the fight and one you walk out of.
In PvP this is close to mandatory. Against mana drain in particular, a capped resist is what keeps you able to act instead of standing there with an empty bar waiting for it to happen again, and it eases the pressure that makes players carry trapped boxes purely to break paralyse.
It matters in PvM too, against anything that casts. Plenty of high-end spawn does more to you with spells than with its weapon.
This is one of the more expensive 120 scrolls, and that price is simply demand — nearly every serious character wants one. Consumed on use, one character, cap raised permanently from 100 to 120.