Let's assume you are a software development firm. Your team members include project managers, team leads, developers, testers, designers, sales persons, marketers, and many more. Let's say you have client details in your work items that you do not want to reveal to your developers, testers, and designers. You can navigate to their profile permission pages and hide the fields that relate to the client information. Following this, only those users with profiles that are not the mentioned three profiles will be able to view client related fields. This can ensure your client information is accessible only to relevant users.
Similarly, you've got client users in your workspace and you need to share with them the progress of the project without having to reveal all the data in your workspace. You can navigate to their profile page and enable entity level view access to specific modules like the backlog, sprint board, reports, and dashboards. Users with that profile will only be able to view the data in the said modules. So, the rest of your workspace data is secure.