Casual listeners can't detect it, but it strongly affects the possibility space for production purposes. For instance, if you're writing happy hardcore music at 170+ BPM with a kick drum on every beat, you will get surprisingly little time between those kicks to work with the bass and other instruments. While this is a somewhat extreme example, even something like the difference between 128 BPM and 138 BPM can change what you're making. A commercial house track produced at 128 BPM may sound like 90s Eurodance when played at 138. Similarly, a deep house track produced at 122 might feel like radio EDM when played at 128.

While you could certainly make psytrance at 150 BPM, it's going to pack a different punch. You'll make different decisions about what works and what doesn't, and then it's going to end up sounding even more different from the rest of the 138 PBM psytrance -- oops, you've shifted genres. Slow it down to 110 and then people aren't dancing to the track quite the same way anymore. Try dancing to the same song at 100 BPM and then at 120 BPM... you'll notice the difference immediately. Tempo isn't everything, but it's much more than just a number.