Walex

They just want to know whether it is worth the trouble to go and write up an offer. You can still have a little back and forth on details after you've accepted a verbal offer, but if there are major disconnects (compensation, responsibilities, etc.) then they want to negotiate that at the verbal offer stage.

Insisting that you accept immediately is a bit of a red flag. Often times companies want to do that to avoid you thinking about it too much. They might do that if they have someone else they can turn to if you say no, or if they want to keep you from asking for more for fear that you might lose the offer.