"Blatantly Off Topic" was created in a time when we envisioned folks becoming horribly lost and asking how to find their lost cat on a site about mathematics.
But as the size and experience-level of the network grew, folks were increasingly using "blatantly off topic" as their default, go-to way to dismiss any user who failed to read their nuanced and ever-growing FAQs in their entirety.
It was felt — and continues to be a sound decision — that if you are going to dismiss a question out-of-hand, you should take a moment to leave a quick note to explain why — which is what the "other" close reason provides; that is of course, unless one of the custom close reasons are a better fit.
And, yes, you should refrain from manually flagging questions "blatantly off-topic" unless you feel there is an exceptional issue that needs addressing. Moderators are not generally here to super-vote a question closed for you.