Traditional computer manuals covered everything from physical maintenance to programming libraries. Although the books were convenient, many users didn’t always want to dig through printed documentation or carry it around. So, as space became available, the man (manual) command was created to put the books on the system, giving users immediate access to the information they needed in a searchable, quick-reference format.
There is a manpage for most commands on your system. There are also manpages for important files, library functions, shells, languages, devices, and other features. man is to your system what a dictionary is to your written language. That is, nearly everything is defined in detail, but you probably need to know in advance just what you’re looking for.