It depends. Mostly I just give warnings for sexual content, gore, and strong language, all just in case. If they are minor things that some people might be uncomfortable with, then I'd just skip them. I personally also skip most warnings related to character's behavior, for example drug use, though I'd probably point out if there was child abuse or suicidal tendencies.
But generally I don't see a point in warning people about some controversial behavior when you have a mature story. I'd just warn about the strongest things that are essential to the story and that might put off or straight up disgust many people, like same-sex relationships, mental control/brainwashing, or rape.
On the other hand, if you're writing a lighthearted story, then it would be better to include even the smaller warnings, like death (yes, it's not insignificant, but smaller and more natural), gore, or heavy drug use. Implied alcoholism though is kinda really stupid, unless you're writing a story literally for kids.
As for if it's necessary and if people are more of less likely to read... well, no idea. I'd say it's as necessary as proper tags - might be useful, might get you some negative reviews if you don't include it, might attract some people, but in the end it's only the most crucial of them that really matter. The smaller things can be thrown out and most people won't even notice.
You can check out
Reincarnated as the Monster of all Monsters and what warnings it has comparing to the shit you can find inside. Just write 'read at your own discretion' and you've no problems.
