Why do I see research being republished in Springer?

Springer (and other commercial publishers) publish things because they expect to make a profit. They only act in the interests of the research community if those interests are aligned with Springer's profit.

Springer is able to profit because most of their sales are bulk sales. Since Springer has a monopoly on certain essential content, they can force their library customers to buy nonessential content.

I don't know what book you're looking at, but it could be a reprint volume. These are, as their name indicates, collections of reprinted works. They are usually reprinted works of some famous person (example), or they might be works which the editor of the volume has decided are worth including (example).

These books will usually make it quite clear the works contained are reprints though, so there should be no confusion over the date the original work was published.

Let me suggest that this is benign. A conference will often make arrangements with a publisher to publish all accepted papers. The original date of the paper is the conference date and there may be some distribution of "proceedings" to attendees and, perhaps, to members of the sponsoring organization. ACM SIGPLAN, for example.

But conferences are on a frequent and tight schedule. Book publishing, on the other hand may take a while to organize, so the promised general publication, available to the public has a later copyright date. Publishers prefer a late copyright date so that expiration comes later than otherwise. And publishing in a book gives things a wider audience, since conference proceedings are extremely unlikely to ever be reprinted.

I will guess that the book references the conference and includes the date on which it was held. If you found the paper in the book, then you can reference the conference year as the original publication year.