The easiest way to see these is to consult a PDF schedule -- you'll have to pick a day and a direction as it looks like they are posting date-specific schedules to account for trackwork. But you can see that on a typical weekday:

These "express" trains run roughly between 10:00-17:00 towards CDG and roughly between 09:00-16:00 away from CDG -- presumably during the morning and evening rush hours there's too much demand at the skipped stops to make this express service make sense.

As Porcepic points out, you don't actually save any time -- an "all stops" train leaving Gare du Nord at 13:00 arrives at the airport (TGV 2 station) 34 minutes later at 13:34, whereas the "express" leaving at 13:12 does the trip in only 29 minutes, but still arrives later at 13:41. You can see from the schedule that trains never overtake each other and the order they are in at Châtelet / Gare du Nord is the order they will arrive at CDG -- so in theory, if you are in a rush, it is always best to get on the first RER B that arrives, assuming normal service (which is not a given on the RER!!)

All that being said, if I'm not in a huge rush, especially when leaving the airport, I also prefer to wait for these "express" trains, even though I'm aware I don't actually save any time. The RER B can be very heavily frequented even midday and it's nice to get on at the airport, find a seat/space, and not have to worry about lots of other passengers getting on/off, moving around my luggage, etc. until the train reaches Paris.