Maintaining Concentration is a super important factor for spell casters. Many of the spells the want to cast require continuous concentration to work.

There are 4 ways a spell caster loses concentration:

  1. CASTING another concentration spell
  2.  taking DAMAGE [DC is half the damage or 10, whichever is higher]
  3.  overwhelming DISTRACTION [DC=10]
  4.  becoming INCAPACITATED (or killed of course)

Luckily, the caster is given a chance to roll constitution saves for reasons #2 and 3. Therefore, it is valuable to invest in constitution as a stat – helps with those CON saves, and gives you extra HP for the times you take damage.