Offhand guess: either lust gain in battle or the ability to use the dildo wherever you are. I'm not very far into the game yet but my experience with such mechanics suggests these might be it.
You only use the dildo in the home bed. And lust gain does not really get affected by anything unless its based around of when you in a cutscene of getting fucked.
I belive it was mostly used so you could continue in the relationships. Because its not for your stats in combat, as Willpower is the main thing relates to sexual struggle, and Allure for making you more likely to get "Raped" from the enemies.
