TY175: Spokes Question

See below, many (but not all) of the spokes I've removed have this spacer on the nipple end, inside the rim. I have Japanese DID Rims if it makes a difference. I'm assuming these are installed is the thread runs out before the spoke is tight. Is that correct? And, as I'm reusing the DID Rims with a genuine Yamaha spoke set, do you think I'll need them, and if so, where can I get some new ones??