A practicality.
Glass at Doddiscombsleigh in Devon.
So you are medieval priest in a rural parish, with very fews clerks to hold your liturgical books for you. What do you do at a baptism with your nice new copy of the Sarum Manual? Well you either use a wooden lectern or have a stone one constructed against the pillar next to the font. That is precisely what they did at Beckley in Oxfordshire, where a fifteenth century stone lectern built as an integral part of a pillar next to a plain reset Norman drum font. There are one or two stone gospel lecterns still in existence, built out from the north wall of the chancel, but this font lectern is, I think, a unique survival.
Comments
In my childhood parish there was a small "missal desk" that was fitted to mount on the edge of the font.
One would expect to find the font at the West end.