Round-Robin Scheduler - What exactly is the quantum time used for?
For the Round-Robin Scheduler the quantum time is to ensure that each process has a share to the CPU and we don't have starvation problems. It's known for being fair, such that each process shares the CPU equality. Quantum time is it's the amount of time spent on each process in the CPU until we context switch to the next process in the ready queue. For example say you have P1, P2, and P3 in the ready queue. And some P4 enters the ready queue at 550ms. The RR would perform something like this:

P1 - 0   to 100ms
P2 - 100 to 200ms
P3 - 200 to 300ms
P1 - 300 to 400ms
P2 - 400 to 500ms 
P3 - 500 to 600ms [P4 enters]
P4 - 600 to 700ms
... [P1 to P4 cycles]


If a process has a CPU-bound execution that is less than the time slice. Then we move to the next process keeping the same time slice. For example let's say P1 only needs to run for 50ms until it's done. Then you have:

P1 - 0   to 50ms [finnished, start P2]
P2 - 50  to 150ms
P3 - 150 to 250ms
P2 - 250 to 350ms
P3 - 350 to 450ms
...