As of now, User with Editor level can make changes to any fields within project. However, in our cases, there are a few fields which should be edited by Project Owner/Admin only. The "custom roles" features would be nice to have to achieve this goal.
Also, if a user assigned to a task, he/she doesn't necessary need editor access to to tasks in the same projects. "Custom roles" should be able to limit user access down to task or fields level, comparing to project level at the moment.