1. If you strum the strings of a guitar and want your laptop to make you sound like Jimmy Hendrix, it had better output sound quickly enough that you perceive it happening "immediately." (I.e., it should sound and feel just like an electric guitar that's hooked up to an amp.)
2. Such a laptop like the one above had better make you sound like Jimmy Hendrix for every moment you are playing the guitar. The website doesn't state it, but they are making a claim to 1ms round-trip latency in the worst case. If your first strum of the guitar convinces you that the resulting sound is happening immediately, then every subsequent strum must also sound like it's happening immediately, too. Users don't merely rant about a sporadically janky digital instrument as if it were your run-of-the-mill Windows 10 UI. They throw it out.
3. There's something I call El Dorado latency, which is always half the latency being claimed and is eternally sought after by users. (Works for system latency, round-trip latency, etc.) Since the unit for measuring audio latency is milliseconds, and since "1" is the last whole number greater than "0", a claim of 1 ms essentially short circuits the search. E.g., if they had claimed "2" then El Dorado latency requires the user to imagine how much better the system would sound at "1". :)
4. Okay, I'm being a butt in #3 above. Round-trip latency in audio seeks to go down to "ultra-low" latency measurements for the same reason gamers want frame-rates to go up to gazillions. In each case it means developers can choose one of the following for their end-users: a) add more hungry, hungry hippos per unit of time, or b) make the same number of hippos hungrier.
Edit: clarifications