I just voted to reopen this question: EXT4-fs error after Ubuntu 17.04 upgrade. Then I noticed there was an option to protect it.
Why would I want to protect a closed question when I'm the first person to vote to reopen it?
Since protection works by preventing users who haven't earned much reputation on the site from posting answers, and new answers cannot be posted on closed questions until they are reopened, it's not very common that it would make sense to protect a closed question.
I can only think of one situation where it really makes sense to protect a closed question: it has many views and it very recently received multiple posts as answers that were not really answers (and that were deleted, or are about to be deleted), and then it was closed, but wrongly, but people quickly realized that its closure was wrong, and now it is about to be reopened. In this case it would make sense--or would at least be acceptable--to protect it even before it has been reopened.
Basically, I think the system allows us to protect closed questions so that we aren't forced to wait until they are reopened to protect them, in rare cases like that.
I presume there are other situations when it makes sense to protect a closed question. I presume also that they are all at least as uncommon as this one. If the ability to protect closed questions is frequently misused, then it would make sense for Stack Exchange developers to remove it and only allow questions to be protected while they are open. If not, then it's not doing any harm; just don't use it except in the infrequently occurring situations where it makes sense to do so.
Note that preventing users from posting answers until they've gained 10 rep through actions on this site--and adding a banner to the question indicating that it is has been protected and by whom--is all protecting a question does. Protecting a question does not change how many reopen votes are required to reopen a question, nor (as you've mentioned) how many close are required to close it again, nor does it affect how any review queues treat the question. Protection also does not affect how many delete votes are needed to delete a question.