Use cases
Construction: Supervisor of a building site provides input to the engineer about the progress of the construction. Engineer cross verifies the updates and adds them to the project. Therefore, it is not necessary for the supervisor to have edit access. The supervisor is granted Read-only access so that they can review the data added, check the tasks allocated to them, and get notified if @ mentioned in any of the comments.
Marketing/ Sales: Freelancers are hired to sell products. These freelancers work under a team lead. Since these freelancers are hired on a temporary basis, giving them full access to project data isn't needed. The team lead feeds the freelancers' sales data into the project. Therefore, these freelancers can be added as Read-only users in the project for them to verify their sales data.
Software development/ IT: Sometimes, a customer would request to view their data in our system. At such instances, we can give the user Read-only access to look into their data.