Because if I were a marketing agent at Tampax, I'd like to do my fellow men a favor and set those directly-sharable-to-your-timeline-and-eligible-for-autoplay videos I'll be uploading to the brand's official page to "women only" because not too many guys care to be shown advertisements for a product that they're not (generally) anatomically equipped to use, much less have a valid need of, because they have one or more of "that friend", or worse, "that family member", that shares the weirdest damned shit, including advertisements for women's sanitary products.
This becomes increasingly important when I'm paying per-impression to have these videos pop up on random people's newsfeeds in the ad slot. I don't want to pay for someone who'll almost certainly never care about my product (guys: do you really sit there and make decisions on which sanitary products your female partner uses? Do you sit there and think about what bit of cotton will sop up the most blood? No, she just tells you what to buy and you squeamishly get in and out of the store ASAP with the damned things, and hope you don't run into anyone you know during it all.) to see an ad that they have absolutely zero use for.
(I know I'm gonna get flamed for this, but it just came to mind as possibly the number one important use-case for gender controls.)