The images that Charlie provided (
here) make it abundantly clear how this door was intended to function and how it was "modified". In its normal operation, there is a double key deadbolt on the bottom and a spring latch on the top. When the door is simply closed, the spring latch is pushed in by the edge of the strike plate then the spring pushes it back into the hole in the strike plate securing the door closed. The lever handle on the inside will retract the spring bolt to open the door but there is no corresponding handle on the outside
so the only way to retract the spring bolt from outside would be with the key. (using a credit card or flick knife would be easier)
The downside of this configuration is that when the tenant goes to take out the trash wearing only a bath robe and slippers, if the door should close behind them they will be locked out. It was probably such a situation that led to the modification.
By jamming the spring bolt in the retracted position, it will no longer operate to hold the door closed. This resolves the problem of getting locked out of ones home in their bath robe but leads to the condition that amanda describes where if the door wasn't locked with a key it would blow open in the wind.
The key operates the deadbolt from either inside or out. It's possible for the lockset to be configured to link the lever handle on the inside to the deadbolt for either locking and/or unlocking the door but this is apparently not the case here.
BTW Kate, the rollers were a deduction based on the earlier outside image and the description of the door operation. There are no rollers visible on this door (though the rollers would be a good suggestion for the current tenants if they don't want the door blowing open in the slightest breeze when it isn't locked with a key).