You may have heard your FTP called your "Hour Power", however that is not entirely true for most athletes. When it comes to putting out maximal power for a full hour, there are other limiters at play other than FTP.
One significant factor at play is your pain tolerance and mental fortitude. Putting your body through a maximum effort for a full hour is incredibly painful and our minds often give up before our bodies would. Essentially, our minds limit us so that we cannot express our FTP as long as your body is physically able to. By following a progressive, structured training plan you’ll gain the mental fortitude and physical stamina to become increasingly apt at handling sustained efforts at your FTP estimation.
Now let's say that you've been training for years and your mind is an iron hammer. Should you test your FTP by going all out for an entire hour?
The answer, in this case, is still no. An hour-long, fully exhaustive effort will have a huge effect on your body's fatigue with a disproportionate training benefit. An effort like this would require ample recovery and can derail a well-structured, progressive training plan due to the inordinate amount of recovery required by all but the fittest cyclists. Now consider the difficulty (and dread) associated with repeating an effort this long and fatiguing every 4-6 weeks. It is just not a good idea considering there are shorter and less strenuous testing methods that will help you approximate the same end result.
Finally, FTP is used to estimate your maximal steady-state power, and this power seldom falls neatly at 1 hour in duration. Some riders can barely sustain this level of effort for 40 minutes while other, more highly conditioned athletes can do it for as long as 70 minutes. Regardless, a progressive, structured training plan can elevate your ability to sustain your FTP for longer and longer durations.
By completing your 8 or 20 Min Test, you will arrive at an actionable FTP that you can consistently base your training on moving forward. Your ability to express that FTP may vary, but it will grow closer and closer to that theoretical "hour power" the more you train
Takeaway: Your FTP may not actually represent your hour power, however, that does not change its utility as a training metric.