That first line is more or less exactly what I posted, is your typical usage of grep, and is very easy to learn.
Sure, you can list of esoteric examples of grep usage but that's besides the point if it's not how people would typically use grep (in my ~25 years of command line usage, I can't even remember one occasion when I've needed `-f` - not saying it hasn't happened but it certainly isn't something I've needed regularly)
Take a look at the man page (linux)
Now -f does not specify the file to grep, it specifies a file where to read patterns from (and they have the same "variable" name there, confusing)Not to mention globbing and other shell escapes (which is not grep's fault, of course, but you might end up hitting in some situations)