Now I don't mean committing to memory the entire spec sheet on each body. Learn the features that are important to your own photographic style.
To give you a good example, a few years ago I had the privilege of being my family's official photographer for my parent's 50th anniversary. I took mostly candid shots, to catch family members at ease, but I also had to do the obligatory family group shots.
I wanted to use the double self-timer feature, as I really hate those "rigid" set up group shots. I knew they would watch for the first shot like "deer in the headlights", but wouldn't expect the second. As I was mounting the camera onto the tripod, I realized that I didn't know how to use that feature. Imagine the howl of laughter from the entire family as the "professional" photographer and camera salesman got out the instruction book.
What they didn't realize was, as a nature photographer, I didn't often get the opportunity to use the double self-timer feature.
I do however, often use the regular self-timer feature for my landscape photos.
It seems that like their earlier predecessors, my present Nikon cameras have a useful feature when using the self-timer.
When you trip the shutter while using the self-timer, the mirror locks up out of the way during the countdown. This duplicates the now defunct mirror lock-up button available on many older cameras. This feature enabled the camera to lock the mirror up to lessen the vibration of the mirror slamming up and down during long exposures.
I use the self- timer for my landscape photographs since I often use a small lens opening, and therefore, a slow shutter speed.
This is what I mean when I say get to know your gear.