In theory, you can use any diaper any time. For some diapers I suppose this is probably true. However, in my experience, different diapers serve different purposes. For example, I prefer a pocket when I'm traveling over 30 minutes because it keeps my baby feeling drier.
First, you should know that Baby "S" is a heavy wetter, especially at night. I struggled for months to find a nighttime diapering system that works for us! My opinions of fitted diapers are based on this fact.
I have found that the best times to use a fitted diaper is overnight. If you have a heavy wetter like Baby "S", you will be amazed! No more changing 3 times per night, no more peeing through multiple jammies, no more washing every day! It is incredible how well a fitted + cover can work overnight!
I also like to use mine when I'm just hanging around the house and we aren't going out. That way, when she pees through it, I can quickly change her. When she's wet and awake, she notices and wants a change right away (she can sleep through the wet at night).