Needed? No - it is ultimately a control mechanism that could also be handled within the team itself, not enforced by Jira. But it is helpful - maybe you have a new team that needs some guide rails. Maybe you open up the board to your organization and want free flowing cards into the board, but a gate on cards actually getting assigned to the dev team without a leader checking it out first. There are numerous situations where it could be applied, none of which are critical, many of which are helpful.