Pooled tasks - what is their purpose?

[upforsin]

Hello,

I have created a few workflows but I still do not understand the concept around pooled tasks (I don't mean group tasks). Why would anyone open a task and take it from the pool if he/she can immediately click "Task done" button? 

I understand that this mechanism would be useful if the time spent on single tasks would be huge (in hours, days), but I suppose on average one task takes seconds, maybe minutes, so why would anyone waste time on clicking the "assign" button?

The same question is about "Save and Exit" button which is highlighted and seem to be very rarely used if ever.