The parish clerk was often witness at a marriage because he had to be present in order to open the parish chest to get the register out.
Sometimes, like James Liptrot at Bolton's parish church, prepared the marriage entry to the point of just needing the signatures adding (including "<name of party> her mark").
A relative of mine was parish clerk for 30+ years in the early 19th century. The clergyman liked doing the form-filling, but he signed as witness, even when there were already two signatures. He lived in a large house close to the church; apparently he was not a farmer, so inherited his money. He is down with the occupation of Parish Clerk on the 1841 census, but he was probably too old to be enumerator - 68, NOT rounded down. It's possible that he got someone else to do the leg work. He HAD been responsible for the 1821 census, along with a prominent farmer.
There are also documents referring to him as being responsible for maintaining (or failing to maintain!) the roads, so there were other parish tasks involved in the job.
