They want to mark an entire post as a spoiler. Not a comment. Useful if you want to post something like an image that contains spoilers, so that image shows up blurred in everyone's feed until the user manually clicks the button to unblur it.
Correct. To get around that limitation, I have seen users on other communities mark a post as NSFW and then include in the title something along the lines of "(NSFW for spoilers only)". Obviously this is far from ideal, as some users/instances block all NSFW posts, and some others set NSFW posts to not be blurred at all.
Yeah, my home Instance doesn't allow NSFW material so I could neither create such a post myself or even view it without logging into another account (not ideal for a moderator).
Best bet is just putting [SPOILER] in the headline (without any spoilers images) and/or use spoiler tags appropriately in the text depending on what the OP wants to post about.