I doubt that the lowering of the levels in Drayton over the summer had any bearing on night fishing anglers although they obviously benefit!
The reason that reservoirs like Drayton, Tring, etc exist is to provide top-ups for the canal system. Fishing, boating, etc are just by-products.
Contrary to popular belief, canals normally follow the contours of the land and actually have a flow direction. When someone opens a lock gate to decend to a lower level, a quantity of water is lost to the level above. This water loss has to be topped up from somewhere and that is where the reservoirs come in. It is normal for such reservoirs to be at their lowest level at this time of year as all the heavy boat traffic has been in the preceeding 6 months or so.