You are adept at spellcasting when threatened or distracted.
Benefit: You get a +4 bonus on concentration checks made to cast a spell or use a spell-like ability when casting on the defensive or while grappled.
When you make a concentration check, you roll d20 and add your caster level and the ability score modifier used to determine bonus spells of the same type.
If you want to cast a spell without provoking any attacks of opportunity, you must make a concentration check (DC 15 + double the level of the spell you're casting) to succeed. You lose the spell if you fail.
Not completely necessary, just really good early game or for characters that do not have the best casting attribute. For example, an Inquisitor doesn't need an Wisdom higher than 16 to be able to access all their spells, since they only get up to level 6 spells.
You will probably need to cast on the defensive at some point, though. Just about every spellcaster will find themselves in such a situation at least once. Though again, it isn't 100% necessary to take; it just helps.
Full Arcane Spellcasters (able to cast spells level 0 to 9) typically won't be hurting for feats, since all they really need is their spells, and they don't have the Base Attack Bonus to be able to perform other forms of combat as effectively as other classes.
Clerics are still casters though. They get up to level 9 spells and all. They just have the Base Attack Bonus and Hitdie to be able to be more combat-like than full Arcane or Psychic casters.
And Magus is a funny one. It was a Hybrid class before they were cool.