I'm about to polyurethane my floors, and all the instructions I've come across (including the ones on the can) say to "maintain a wet edge".
To elaborate a little more. When you apply a finish like poly or even paint, it doesn't just instantly go from a liquid to a solid after an hour or whatever the dry time is for the product.
When it's exposed to air it slowly starts to solidify. As this happens the physical properties of the finish change. Most importantly it's workability changes. If you brush on a section of poly and it goes on smoothly it will eventually level itself out so that there are no brush marks.
In the middle of your project the phone rings and you step away for 10 minutes. In that time the finish has begun to set up. If you try and continue the job by brushing where you left off as your brush goes over the partially dried finish it can gouge it leaving brush marks or possibly lift it off completely in some spots.
Most finishes don't like to be over worked because doing so compromises their ability to level and possibly their adhesion. Your brush should only go over an area that was recently brushed or allowed to fully dry if you can't finish what you started.
To paraphrase what Les said and Chris Cudmore said Wet on wet > Wet on Dry > Wet on partially dry. You can minimize the effect of banding when doing wet over dry by feathering the new finish over the old a bit.