There can be cases where multiple resources are assigned to a task. This can be understood if you keep in mind that resources can be people as well as equipment (and material too, for the sake of completeness).
As an example, a task in a construction or manufacturing project requires a specialized joint which can be done using a specific machine. Other work resource, assigned to this task, can prepare the material in lets say 4 hours and then require another 4 hours to join it using the machine. So one resource is used 100% and the other is used 50%.
Similar concept can be used in Software Development projects. Consider a web application which is used connect with remotely deployed specialized equipment (let say on-field spectral analysis devices) and change its various configuration settings. Now consider that during active development there are two testing/qa task, one to verify backup frequency of the device (set from web control panel), and second task to verify the data upload frequency of the device. Two QA team members will be assigned (one for each task). However due to limited availability of on-field device, it will be shared. Each QA member can start his/her task with testing the web interface first and then complete the end-to-end testing on the device. So for each task in the project plan, we'll have 1 person assigned 100% and the equipment assigned for 50%.
Another possible example, where there is no involvement of equipment could be, a safety/quality inspector who spends 2 hours daily on each of the 4 different project sites. At each of the 4 sites, one person is assigned 100% to his/her task and the inspector assigned 25% to those.