1) The common wasn't for pleasure- it was common grazing ground- sometimes used for fairs.
2)It may well have been allocated to the parsonage as recompense for the reduction in tithes caused by enclosure,(not to keep it out of the landowner's hands) and for Northampton, eighteenth century enclosure as opposed to sixteenth or other century enclosure is highly likely. However, that in itself doesn't explain its function in the early 1800s.