Most of the Help menu offers option to search through documentation, support website link, license and registration options etc. In order to search through the help you may need to find if the software has help file. Sometimes it also has documentation link. So in order to search through software, you have to find such link.
The help menu is designed to help people who are not aware of how the software works. Or some people are not aware of the tools and the names within the software. In such case you have to find something about feature in software, in such case such tool allow you to type the name in documentation search bar. And you get the information on how to use that feature. There are many other ways the helpbar can be used. Microsoft based products are allowing search bar as command console where you can even type the shortcuts and work within the app.
So different software has different way of handling the search and help documentation.