Say you have rgb 85, 186, 146 or #55ba92
Firstly, without looking it up, what colour is that?
How bright is it? (you can get a rough idea)
What kind of saturation is it? (again, rough idea)
Now, I want to lower the saturation and keep the same hue.
What about changing the brightness while keeping hue? 
HSL. Knowing that Saturation and Lightness are in %, and Hue is from 0 to 360deg and that each main colour is every 30 degrees (0 red, 30 orange, 60 yellow, etc), I can tell you exactly what colour, saturation, and brightness hsl(86, 80%, 60%) is very quickly. It’s a green with a bit of yellow, and saturation/lightness are obvious. Changing the values is child’s play. Need a bit more yellow? Just bump the H down a bit. Need another hue with the same S and L? Just change the H.
HSL is human readable and editable, once you know how the H works. It’s also extremely simple to dynamically create via expressions
RGB you can kind of guess what colour it is, but editing it is impossible, especially if you are trying to maintain hue. And dynamically creating it with an expression? Forget it
Consider that in UI development, you're mainly finding hues to use, and then playing with their saturations and lightness to get the right colours. HSL plays perfectly into that, along with all the other benefits above