Anyhoo, honestly I don't understand why simply showing up during your ride window is a huge deal. I can't imagine why you would get a fast pass and then want to use it 4 hours late.
There are numerous reasons, far too many to mention, but I will cite a few:
a) The return time is for say, 10 AM but the ride is still a total walk-on in standby at this time. Using the FP during the return time in this instance is essentially throwing it away, since it won't actually save you any time. Use standby again, take your walk-on, and save that FP for later in the day when the line will likely be significantly longer and you may be in the mood for a reride.
b) You start the day with Space Mountain (or Everest, Rock and Roller Coaster, etc) and ride multiple times in a row right after rope drop, while the ride is still a walk-on. While standing next to the FP machine first thing in the morning, you get a FP for later just
in case you decide to ride again much later in the day, when the lines are much longer. Maybe you will make it back to this attraction and maybe you won't, but if you do finish everything else you want to do in the park to make it back for an encore much later, that FP acts as an "insurance policy" that a potential 90 minute wait will be just 5-15 minutes instead. And the reason you don't go back for your "1 hour window" is because you already just rode the same ride 2 or 3 times less than an hour ago, and at this point you are in another part of the park, eager to do OTHER things.
c) You are visiting with family (or in my case, they are visiting me). They aren't "early birds", but if one person in the group (ie, me) is, you can go to the park at rope drop and have them meet up with you later. Using YOUR ticket, you can get a FP for 1 or more people in the group who isn't there, one at a time, until you have one for each person in the party. Because you only used your ticket to get the passes for said attraction, they will all be for different times, but since they've always allowed the passes to be used anytime between the first time and park closing, the entire group can ride together, when the others in your party/family/friends finally make it to the park and the time for the last FP ticket opens up.
d) You are at Hollywood Studios. This, more than any other WDW park, is a show park, and seeing all the shows you want to see in one day already means doing a lot of backtracking to make sure you can line them all up and get them all in. Beauty and the Beast, Playhouse Disney, Little Mermaid, Indy, Lights Motors Action, the 3'o clock parade, American Idol, Jedi Training Academy, etc, are all things that take place at set times. Once you figure out a strategy to make sure you get all the shows you want to see done that day, you may often find yourself with times of 15 or 20 minutes to kill between shows, which isn't really enough time to guarantee doing any of the attractions at that park, even with a FP. But since Disney allows "late" arrivals, if your 1 hour window occurs during a period where you are getting all the shows done, you may indeed find it beneficial to save that FP for much later in the day, when it won't conflict with any of the showtimes you want to see.
These are just a few obvious examples I can think of off the top of my head, all based on personal experience. I could list more, but you get the idea