to clarify it for you and for others that seek this kind of info:
an addon is a wrapper around an existing program, almost all of those programs existed before the wrapper was created.
what does the addon do?
it makes that the program can easily be installed on hassio and in a lot of cases it also makes sure that some settings are already set. (with in mind it can easily communicate with HA)
so it is (also) there to do the things that you normally need to do yourself (configure settings)
when you dont want to use hassio, it is most of the time that you want more freedom. you want to be able to put things together yourselve. so you normally DONT want someone else to preconfigure your settings.
so if you see an addon you like, then find that program and install it “the normal way” and configure it yourself.
addons are created for hassio, like microsoft office is created for windows.
as it in this case goes about tasmoadmin, you go to the github from tasmoadmin and you find there a few ways to install that program: docker, hassio addon, windows and linux.
the normal pip install way would be the linux install, now you have changed to docker so you choose the docker install, and for someone with hassio it would be the addon.
i hope that makes things more clear.