Why does my boyfriend like it when I suck his fingers?

Months ago when we were cuddling in bed, he asked if it was okay if he put his finger in my mouth. I reluctantly said it was fine. I didn't understand why he wanted to do it but I let him anyways. My mouth was wide open with the tip of his index finger in and he asked me to close my mouth with his finger still inside. Is this some sort of kink or fetish that I don't know about? Every so often, he asks if he can do it to me again and I let him every time because I'm not initially bothered by it. Many times while touching me over my panties or when I play with his parts over his underwear he'll put his fingers in my mouth and have me suck them. Is there a reason why? Does he enjoy this????