question on PVC fitting

I am doing plumbing on a basement bathroom sink and on the same wall a utility tub behind it. I went to a plumbing store who employ retired plumbers and he set me up with the fittings I needed. At first I thought the sinks would be back to back, but they are offset by a stud width. He gave me what I think is a SPIG adapter, which is just a short sleeve that fits inside 2" PCV. The inside of this thing is not a perfect cylinder, looks like it has a step or something inside it. I'm not clear if he gave it to me just to join two sanitary tee's together or if it had some other special purpose. I was thinking he used it just to close couple two fittings. As it turns out, they are far enough part that I just use a piece of 2" to do the same thing. Am I missing something by not using this adapter?

Thanks!
Eric

It sounds like you have a 1-1/2" coupling. Using that inside a 2" pipe would not be correct.