The idea of this flow label field is that it may be set by applications to indicate the fact that some traffic is to be considered belonging to the same flow even if the application is exchanging the data without making use of a TCP connection (this would allow to identify the integrity of the flow by looking at TCP port numbers).
There are no rules how an application has to set the flow label field, it is completely left open to the programmer. Routers could one day make use of it, however since there are not yet any applications using it, it is not yet of much importance.