When they are useful.
Everyone’s different. I’ve read books where I skipped the torture scenes. The fact that they are there? Tells me more than I need to know about the characters involved. I don’t need to see it play out. Its purpose – to show me a characters suffering and another characters cruelty – is fulfilled merely by a glance.
Some other people may like to sit through all of that. Maybe. But, if all you showed was torture scenes, if your book was nothing but gratuitous pain? Well two things are going to happen:
- Your readers will get bored (seriously, even an entire book of sex scenes is a as dull as dishwater unless each of those scenes has a different dynamic and means something different.) It’ll just be “oh, here’s another of those scenes”
- They will see that there’s no point, and they will leave, because generally speaking, people don’t sit through things they don’t like.
Now obviously the second point applies (hopefully) more to violence than to sex scenes. Sometimes, if they’re written well, people will read sex scenes because they like the sex scene. This is fine, although be aware of what genre your book is in, and what you’re promising to the readers by using that genre as a tag.
People will get a bad case of “can we please get onto the plot” if you have too much of the same thing going on. Even sex.
This is true even in erotica. Maybe if you don’t read it much, a whole book of sex scenes where emotions and outcomes never change and the actions don’t affect anything will enthral you. I don’t read a lot of erotica, because I find it difficult to find books that do it well, but I’ve read a fair few romances with high heat levels. And I find it difficult to not skips sex scenes after we’ve reached number 4. Because things start to repeat.
Everyone has had sex. If nothing important is revealed or happening during a sex scene, eventually people will skim over said sex scene.
Which brings us full circle regarding relevance to the plot. Sex can be relevant. As can horrid things like torture, death, rape. But as to how much of that needs to be in there?
It needs to be there if it serves your book. If the scene shows character, or emotional change, or plot elements. If it doesn’t? Keep in mind that shock factor only lasts for so long, and eventually people will get tired of reading about the same people having sex over and over.
Even in these circumstances, you have to give readers a reason to care.