Open work permits are important because the alternative is an employer-specific work permit that locks someone in and ties them to one employer, with one role. That employer can use that power advantage to exploit the worker. They know that the worker can work only for them and that they're relying on that job for status in Canada. They can then say that they need to work under very abusive conditions, which no domestic worker would ever work under, and that worker has very little means to change their employment. It's a challenging process and a very lengthy process.
Many workers will continue to work for the abusive employer simply because they don't have any other choices. If they had an open work permit—