Why is the default value of the concurrent setting 1 ?

I'm really unsure whether or not it is appropriate to open an issue for this; please close if not.

So i noticed that my jobs even if they are in the same stage would never run in parallel... I wonder why and quickly found this. The default concurrent setting is 1. Why ?

This seems pretty weird to me. I have a server with 10 runners registered on it; why can't they work at the same time. So basically this setting make it useless to register multiple runner on the same machine.

To me it looks pretty odd but I guess there is some reasoning behind this default value... What are they ?