If you only need the value in this bar, do not use series.
If you will need to retrieve an older value (i.e. a value from bar 4 while in bar 0), use series.
For example, a SMA Period = 5 calculation calculates an average of bars 0 thru 4, so it requires a price series as an input, but its output is a var. But if that needs to be a series for yet another calculation, you can plug it into a series.