Imperative time allows you to enter time value as per your consideration. For example you can enter 1 for 1 hour, 0.5 for 30 minutes, 0.25 for 15 minutes. :) Without imperative time it will restrict you to enter .59 at max i.e 59 minutes.
The idea of expressing 15 minutes as 0.25 is useful. “Imperative time” is a terrible name and I suspect the product of some misunderstanding or translation error. There should be a slider button and it should say “6 min/0.1 hrs” or similar.