The Concentration skill has two functions.
Spellcasting. Allows you a chance to continue casting a spell after being damaged or otherwise interrupted. When you take damage you roll a D20 + your Concentration score against a difficulty class of 10 + one tenth of the damage received + the level of the spell you are trying to cast. This check occurs when either casting from spellpoints or using a scroll.
The Quicken metamagic eliminates the check when casting a spell from spellpoints while it is active, however, as scrolls cannot be quickened and Quicken costs spellpoints, the skill is still useful even with the Quicken metamagic.