I can't understand the grammar and meaning of this phrase:
a single worthy cause – spread too thinly
What does it mean generally and what's the use of "-" in it.
The complete paragraph is:
Unlike their Extraverted cousins though, Mediators will focus their attention on just a few people, a single worthy cause – spread too thinly, they’ll run out of energy, and even become dejected and overwhelmed by all the bad in the world that they can’t fix. This is a sad sight for Mediators’ friends, who will come to depend on their rosy outlook.
UPDATE: I found another example of using dash:
In the 1939-45 strand of the story, Turing has grown up physically – though not, the film implies, emotionally.
I think it says: Turing hasn't grown up emotionally and the film implies this as well.
But still using dash doesn't make any sense to me. For example, I expected another dash following the first one.