Your question is valid as many guests will not understand the inconvenience or the risk of "just dropping off their bags", and will consider any fee a rip-off. If you accept a fee that means you can do it, why not do it for free as part of the service?
I've been through this and it is a big hassle. Once you've opened the door and inch they'll take a mile. Had someone wanting to 'just get in to drop off luggage" *while the prior guests were still there at 8 am in the morning*.
- I would make it very clear when stating check in and check out times also say "We can't store luggage on arrival or departure day".
- Maybe give a discount for booking an additional prior night or subsequent night if they want to arrive very early or leave very late.
- I always ask for flight info so I can see if this is going to be an issue. If their arriving flight is early or departing flight is late I propose extra night at discount fee, if they don't want it I make it clear again we can't store luggage and can't guarantee earlier or late checkin-out.
- Of course if the apartment is clean and ready, or if I don't have same day turnaround on departure day, I let them arrive early or stay late for free but this is only confirmed the day before and depending on my cleaners workload and schedule.