That's an interesting angle! Without your clerk's comment, I'd have agreed that the reserve is to cover unanticipated expenditures. Insurance will cover straightforward risks, but it's quite possible for situations to arise where the council wants to act, despite a lack of relevant budget. Another reason for it is that budgets are always only estimates and a cushion is needed to cover any variations. It is not good to add provisions to budgets - they tend to result in an ever growing reserve.
So far, I've never heard of a principal council failing to hand over the precept promptly. But I have no experience of dealing with the apparently growing number of authorities declaring themselves effectively bankrupt. So I don't know whether there may now be a greater likelihood of disruption of routine payments.
The usual advice is for the general reserve to be half the annual precept. You can also hold earmarked reserves in addition to specific future purposes.