To prevent double bookings.
If more than one service or event uses the same physical object or space. For example, you only have one tennis court but have multiple tennis instructors and have open play pickleball. Add the tennis court as a resource. If instructor Steve is having a lesson at 2pm, then nobody else can book the tennis court at 2pm. Your single tennis court will not get double booked.
To simplify your services.
If you have multiple of the same resource. For example, you have six tennis courts. You allow tennis court reservations during open play. Create six resources called: Tennis Court 1, Tennis Court 2, etc. Then create one service called: Tennis Court Reservation. Your members can reserve any of the six courts until they are all used up.