My initial idea is different specs so that not every iteration of the same job will be 100% the same.
For example, a PLD spec who trades mitigation for a better offense, for people who prefer off-tanking, or a spec that improves your healing potency, etc...yet the three spec would still be tank specs.

The existence of jobs and classes point in the direction where they considered different roles between the class and one of its job, but, apparently, SCH is a "proof" that it doesn't work well, even though SMN and SCH are very efficient.