Writing opinions which may be either concurrences or dissents is a long-held tradition among the Associate Justices, and there is no requirement that all the Justices speak with one voice.
Concurrences or dissents may prove to be very useful in those instances where the Court is compelled to revisit an earlier opinion. These opinions are very much like the Federalist Papers in explaining the Constitution, insight into what the reasoning was at a given moment in time.
It is particularly useful when the separate opinion provides crystal clear logic that is missing from the original position, like, well, it's a tax, man.
I suggest everyone read Byron White's dissent from Roe v. Wade.
It is simple, concise, and to the point as to why the court got it absolutely WRONG.
It is based on the Constitution.
In Roe, the majority simply made it all up.
Blackmun's "opinion" is just that, an opinion.
He could have made it a simple "yes" vote, but he went off the rails with creating a "trimester" system.
That wasn't even part of the argument!
My lawyer friend called it: "OK, here's my ruling. Now get out your pencils, because I'm going to make up some serious bull$hit"