I read about SP accumulating something just sitting there but I still don't know if and how this works. After a certain threshold perhaps.

About 16% of the reward pool is paid in interest to Powered up Steem, there is no threshold. If you do nothing, you will see your wallet tick upward.

Anyway, it's the power to curate, i.e. support people and that on itself is great, so that's why I prefer turning almost everything into SP and I wait for the times when my support is going to matter. Will be awesome :)

I remember when mine was making a lot more difference, it felt great. However, the interesting thing is that at a Steem level, I am distributing more now. :)