To start this list off, one of the mandatory tools in photoshop as well as illustrator is the pen tool. Whether its to make custom shapes or trace over objects its utility is endless. Watch this video to understand all the main features as well as more in depth techniques on the pen tool.
Finally as an addition to this video recommendation thread you might be a more advanced/experienced designer that has some knowledge on all the shared subjects. Even if so I'd recommend this video to all levels of experience for these 10 things photoshop can do. You might have missed one, doesn't hurt to try.
Please comment/like below if you want me to try and make this a weekly or monthly thing. Also feel free to state if it helped.
Please try it as an monthly thing this will help people develop Also the first video really helped me so I guess that is a plus, Ninjaibis has some good helpful vids