At the risk of boring you with repetition, the reason we cut and remove the grass is to reduce the fertility, so that coarse grasses, thistles and docks are given a check. This in turn will heighten the chances that other wild flowering plants will thrive. We want more wild flowers and plants
(a) for their own sake,
(b) to provide food for beetles, bugs and other insects,
(c) as a resource for a variety of bees and other pollinators.
In short - to increase biodiversity on the site, which might then spread to other places nearby.