Quite a lot, for they are the nearest we have to a right to some land, apart from the occasional common rights. Access to land is an important requirement for liberty. It enables local, shared control, as well as a more diverse life incorporating small industry with cultivation of the land. Allotments represent a 'right' to land in the sense that the Council is legally obliged to provide them, as long as there are more than three people who demand them. However, there are often long waiting lists, so it seems that Councils don't feel obliged to provide them to all applicants.
Allotments are a way of becoming more independent of the system.
Growing your own vegetables, and becoming involved in the local informal economy by exchanging produce and skills represents a potentially undermining independence from the system. It needs us, and if we reduce our dependence on it, it may flounder. More individual access to land would greatly facilitate small scale organic agriculture. Having the use of some land has been a symbol of freedom for a long time. For many this freedom was lost when the enclosures turned land into a commodity to be bought, sold and owned.