Supermarket loyalty cards

I need money/shopping/retail experts to comment on supermarket loyalty schemes. 

Comments should cover: 

Are supermarket loyalty schemes worth signing up to?

Why are supermarket loyalty schemes important at the moment? (i.e. cost of living, food inflation)

Which loyalty scheme offers the best rewards?

What do you need to watch out for with supermarket loyalty schemes? (i.e. points or vouchers expiring)