Making labels and data sheets, numbering stakes, and measuring out products are examples of things which may be easier to accomplish in the laboratory than on site.
Although these things may not seem to be very time consuming, they actually are, and if you can avoid doing them while standing in the sun or wind, or while your field helpers are waiting to get started, it will be time well spent. Four people standing around for 15 minutes while you write labels or number stakes equals more than one hour of lost time.
It's unlikely that any field trial will ever go off without a hitch and rapid decisions must be made. Some people will hesitate to make a change because they are afraid of being wrong. Over time, as one gains experience, one's decision making process will improve, but no field trial will ever be as perfect as research conducted in a laboratory environment.