The justices are required to rule on actual situations, not abstracts, and so very often their decisions are strongly related to the facts of that situation. It’s not all Brown v Board, and “seperate but equal” being inherently unconstitutional. The process of putting a decision together is a process of coalition building. in a 9-0 decision, for example, that is an easy process, everyone is essentially on the same page. The decision can then be written broadly, to apply in many situations, and everyone is signing on.
When you get to closer decisions, however, the need to get that 5th justice on board limits the breadth of the decision, and so limits the degree to which the case is important precedentially. These days, its a matter of restricting it often so Kennedy feels he can sign on. Previously O’Connor was often in that spot. So, in order to bring the 5th justice into the coalition, the decision tends to get watered down - in particular, it becomes heavily based on the facts of that given situation. While it is still equally precedential as a 9-0 vote, the 9-0 decision, because of the way it is written, is more likely to be applicable to a broader range of future factual decisions. The 5-4 decision, on the other hand, has far less future effect, as it it likely to be very fact specific. A good person to look at on this is Richard Posner - in his decisions on the 7th Circuit Court of Appeals he enjoys seemingly going against SCOTUS precedent, and will often base it on factual distinctions.
Also, future courts are far more likely to overturn a 5-4 vote than a 9-0 (I guess, but I bet there is research on the power of stare decisis in split decisions).
Hope that is clearer. I could express it better over a beer or seven if you were buying.