If you regularly volunteer for the same task then you are helpful. A one day and never seen again probably wasn't helpful.

I've volunteered one offs before. A local urban farm needed to plant dozens of trees. They spent 15 minutes teaching them turned loose the group. We got them planted in like an hour. Then we moved onto weeding and litter cleanup.

Yes, I volunteer for something that requires a week of training. I volunteer a day every month or so.

Well, at least you recognise that you're doing it for yourself; self-awareness is the first step. Maybe you could donate money too, to make up for it.