You don't need the spacer. It's just there as a convenience for those who want the wheel farther forward, and usually used in pairs. Also on many bikes the wheel doesn't center correctly if the wheel is all the way back on the dropouts, you're just a bit lucky that way.
If it works for you this way, you're set. If you decide you'd like the wheel farther forward, and have the convenience of automatic centering, buy another spacer and set both for centering in the right position.
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