Git rebase is used to make the branching paths in history cleaner and repository structure linear.
It is also used to keep the branches created by you private, as after rebasing and pushing the changes to the server, if you delete your branch, there will be no evidence of branch you have worked on. So your branch is now your local concern.
After doing rebase we also get rid of an extra commit which we used to see if we do a normal merge.
And yes, one still needs to do merge after a successful rebase as the rebase command just puts your work on top of the branch you mentioned during rebase, say master, and makes the first commit of your branch as a direct descendant of the master branch. This means we can now do a fast forward merge to bring changes from this branch to the master branch.