For example, I have an electric piano that I can connect to my computer and play. I had to install a special software called "jack", because PulseAudio would have me wait before I could hear a note I played.
Jack was not simple to use, but even if you were good at it, there were problems. Imagine you wanted to play along with some music. Well, bad luck, some software do not support jack, and you can't use it together with PulseAudio.
Now lets say I want to stream my playing on Twitch. Again I will have trouble, because the many moving parts in streaming, including video, make it very difficult to output a high quality stream.
Last, when I use DaVinci Resolve to edit and upload my stream to Youtube, I want to record some voice over. Resolve has this function called "ADR", or "Automated dialogue replacement" -- it's a feature they use in film to record the actors on the computer. I could use that to record my voice over, but ADR on Resolve doesn't bother with either Pulse or Jack, and uses something called "ALSA". That means that until Resolve finishes using it, I can't use my microphone!
With Pipewire I just click and it works.