Doing this decreases certain types of latency in certain situations. As an example, it tries to have interrupts disabled less frequently and for shorter intervals, and uses mutexes instead of spinlocks.
As a result, using linux-rt can provide a lower latency experience compared plain linux.
The reality is that "real time" as a definition covers many "features" and design choices because many ducks need to be in a row for real time tasks to run properly. Decreasing variation in the scheduling of (real time) tasks is one of those items.
As a result, it's entirely reasonable to call "linux-rt" "linux-rt".
Is extremely desirable. Those multi-ms peaks of latency Linux has are the ones that cause audio cuts and perceived hiccups.
Of course it doesn't matter perceptually if the average is 1µs or 5µs. It's all about the peaks, and keeping them bounded enough so that latency does never cross the perceptual threshold.