Using automatic exposure locking
Back in 2008 when I set up my D90 and wrote down my settings, I said this about the AE-L button:
I don't know enough to use exposure holding, but if I do 'tap to hold' seems to be the least obnoxious way of doing it.
Boy, I was kind of innocent back then. Nowadays I have learned what autoexposure locking is for the hard way and I use it reasonably frequently. The simple way to put it is that locking the exposure is the quick way to deal with the importance of watching your exposure from shot to shot.
If you're not in full manual mode, the camera can change the metering during a sequence of photographs; it can do this even if all you're doing is changing the exposure compensation. Locking the exposure with AE-L counteracts this, giving you a stable exposure that you can make consistent adjustments to. Otherwise your attempts to adjust the exposure to get the picture right can be happening on top of quicksand, so that you dial in some negative exposure compensation to correct things but then the camera decides to expose more so in the end your exposure winds up just the same. This is both pointless and frustrating when it happens (and very puzzling if you don't notice the exposure shifting on you; here you are adding exposure compensation yet nothing is happening, or the wrong thing is happening).
I've repeatedly stubbed my toe on this so these days I've learned that if I'm taking a sequence of pictures of the same thing the first thing I should do is hit the AE-L button, especially if I'm just working to get the exposure right. Otherwise, even if I'm just lowering the camera to look at the histogram after I've taken a picture the composition can be just different enough when I bring it back up to my eye that the Nikon matrix metering changes the base exposure.
I maintain that my choice of 'tap to hold' is absolutely the right option for this on Nikon cameras, at least for what I want to use AE-L for. It would be very hard to have to keep one finger on the AE-L button all the time, even when I'm doing things like checking the histogram for specific areas of the picture.