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I think their role is changing. Before Corbyn, their role was to help campaign for their MP during elections, aid the party financially with their membership fees, and, of course, vote for their constituency candidate from a group of candidates chosen by the party. In my constituency, they ran campaigns to save libraries and stop damaging developments. Their role now seems to be to dictate policy, choose the leader, and threaten with de-selection any MPs that don't agree with that leader and policy. So the focus of the party is moving away from a grounding in electoral success.
Edit, another important role for Members of course is to protect working conditions and jobs through joining the afilliated trade unions. A clear example of this trident, where Corbyn has sort of given in, though much of his new membership would oppose it.
Yes, but 170 of them have no confidence in him. Not just one individual. Together, as a group, they have a bigger, and more crucial mandate.