Ok a reason to do that is because some users may be the only one editing the code for the repo and so need to review there own code like they can in gerrit.
For test plan not every repo needs a test plan and I doint think we used it on gerrit.
Just to be explicit: You can self-merge and push changes already (if you have push permissions, obviously). You're right though, I don't see a way for the diff author to self-review their own diff other than by just leaving a comment. This is discussed more in-depth in T138.
The making of test plan optional per repository was discussed upstream at https://secure.phabricator.com/T9331 and is closed as won't fix.
As there does not seem to be anything here not already discussed in T138 I will close this as a duplicate of it.