There are a few different types of attic designs. One is insulating the floor and venting the space above the floor with soffit (eave) vents and an attic fan or two. One reason for the air flow that hasn't been mentioned is that, since the hot air will exit out the top of the roof (whether the attic fan is powered or simply turned by the force of the hot air rising), then it has to be replaced by something. If your soffit/eave vents are plugged up, it will suck air out of your house, which, of course, is not good for your electric bill.
There is another type called the CATHEDRALIZED ATTIC, which is where the insulation is put on the underside of the roof itself. CATHEDRALIZED ATTICS SHOULD NOT BE VENTED. Since it sounds like you have cathedralized your attic, you should not install soffit/eave vents or baffles.