Gun cleaning and field stripping
I’m by no means a gunsmith, but I have found that I know my firearms better than many and that’s because I took the time to learn how to deal with gun wear and gun maintenance. All firearms will eventually have something break on them, or the springs in the trigger or recoil spring will wear out. I have a few 1911 45acp guns that will get really picky about ammunition once they hit about 4000rds through them because the slides will get weak and some of my handloads that are not as clean as factory ammo fail to go into battery.
I think it’s a good idea to know and have replacement parts for your firearm at least in your home. There have been plenty of range trips that have been spoiled when somebody has a malfunctioning firearm or breaks an extractor. There are a few gun parts kits you can keep with your AR15 that will only take a few minutes to replace if you ever have a breakage. Glock parts are easy to replace, probable more so than other firearms because all you need is a Glock tool to punch out a few pins and you can easily replace the firing pin, springs, trigger ect.