Sometimes during project budget proposal requests, bidders might only want to show indicative dates in their bid schedules to the client and an easy way to do this is by showing only the month and year of a task in a Microsoft Project schedule.
Another scenario for displaying schedule dates in mmm-yy (month & year) format is when the schedule is a summary roll-up (Level 1 or 2) for use by senior management in discussions or presentations and the request is to keep things high-level.