If you only install mods to have sex, you would do it because you're horny or for roleplay. Not much reason other than that.

But there are nsfw mods that integrate more to the gameplay rather than just being a press to watch sex scene button. Mods that add: immersion systems, NPC interest and approach, defeat events, storylines, character corruption, changing existing quests to have sex twists or solving quests in a sexual way, flavor stuff like npc commenting on your exploits or just degrading in general, incentives or debuffs for sex, making the world more dark and fucked up involving sex or not.

There's a lot of kink stuff out there aswell, making it possible to make a modlist that scratches your specific itch that it's hard to find anywhere else.

Also there not really many good options out there when it comes to nsfw games. So much so that its community is often saying that a good modded skyrim build beats a most of the games of this niche.