Why using git commands instead of githhub's web page?

The short answer is there is no difference - you're doing the same thing with either method :) However i would argue that it's better and much easier using the git commands for a couple of reasons.

First of all the changes you make will be made locally first when adding and committing the files, before pushing it to github and so both your local tree and the tree on github will be the same which will prevent any conflicts and IMO there would be less of a chance of you forgetting to add a file.

Secondly it's waaaay faster to add and commit your changes when all you have to do is write a couple of commands as opposed to clicking and dragging each individual file. This can be a pain in the neck if you have multiple files whereas with the commands you can add all your changed files with a simple git add ., then git commit -m "Commit Message and then push it to github. It's also handy to use git's other commands like git status and git diff to track your local changes before committing.

One thing i do as a habit is - i create the repo on github, with it's description and whether its public/private etc and then clone that locally - i just prefer doing it that way, so really it's whatever suits your needs :)

Hope that helps!!