In what application niche is parallelStream from Java useful?

This looks like a nice explanation of cases of where and why. https://computing.llnl.gov/tutorials/parallel_comp/#WhyUse I personally see no interesting cases in user centered web applications.

The fork/join Framework is a really cool low level api. Many other higher level frameworks use it under the hood very successfuly. I've used it for test data generation. Cache bootstraping. Data processing etc... In many cases you get a really good boost of performance in others its just unnecessary overhead.

A similar question is asked in Should I always use a parallel stream when possible? Note the second answer is given by Brian Goetz, a Java language architect at Oracle who was involved in the design of the Stream API, so his answer may be considered authoritative.

Top answers are quick to point out that parallel streams include additional overhead necessary for coordination and thus will only increase performance in scenarios where the amount of individual processing per stream is significant enough that the gain from parallel processing overcomes that initial overhead.

Unsurprisingly, as with any question of performance, the advice is to measure rather than guess. Start with a sequential stream, and if you have a large number of elements each requiring complex computation, measure the performance difference of switching to parallel streams.

Additional guidelines, such as those listed in the OP, may be helpful; but people are notoriously bad at identifying performance bottlenecks, so any guidelines are likely to fail eventually in the face of actual measurements.