Code can be read without any syntax highlighting or text decoration, obviously. But adding those things is an additional information stream that makes processing faster and more reliable (redundancy in general has this effect).

As you said, it is especially useful for making certain code smells instantly visible at a glance.

I also find that different kinds of code will get different "color rhythms" (e.g. low-level algorithmic code vs. high-level code that calls a lot of functions vs. code that does a lot of operations / mutation of object or class properties) when syntax highlighting is properly semantic. This makes scanning for certain types of things (where objects are being mutated, where variables are introduced, etc) extremely fast, since you don't even need to read the characters.

I also find that rich syntax highlighting makes the codebase easier to remember, since the color (along with things like the line-lengths) gives each function a sort of unique visual texture.

Of course, all of this is personal preference. I am a very visual thinker so this kind of stuff helps a lot for me. Some people are far more verbal in their mental imagery or may remember code chunks solely based on semantics. Then, obviously, a bunch of color and/or text decorations might not matter much, or even just be a distraction.

In my usage, the coloring allows my eyes to “snap” to relevant chunks of code and restrict scanning to the bounds of those chunks.

Functionally it seems similar to spatial memory where landmarks are used as navigation shorthand and is impaired in circumstances where everything looks the same (e.g. in one of those sprawling suburbs with 100 of the same house).

I remember colors better than I remember names. It's a difference in how I process text, and for code where there's already 'types' of syntax, colors help differentiate between all those. It's not that I can't find the for loop, it's just that visually it becomes more distinct to me if all loops are a certain color. And class names are a certain color. And those colors tend to stand out better on a dark background than a light one. Reading light themes, or even unthemed code just feels like an additional chore that's solved pretty simply by using a dark theme.

No one needs it, but it's nice to have. It's the difference between being partially or fully colorblind or not being so, in terms of being able to distinguish parts of the codebase more easily. That is what the parent means by "easier to parse code."

> I like cursive though to highlight variables that are assigned more than once

Exactly, everyone is different, as personally I'd hate cursive and ligatures. Think of it as, what works for you doesn't work for everyone, and what works for them (color) doesn't work for you. But let people have their preferences.

I'd need a citation for that statistic, and I'd also need to see which themes are actually used.

> IMO, the best themes do typically have minimal/functional highlights, which results in more text that is the base color

And IMO, those are the worst themes.

These things are just preferences, but it is an objective fact that a good highlighting scheme makes certain information immediately visible, without requiring the reader to parse the actual characters. Whether or not this information is something you find helpful or annoying depends on your processing styles and preferences.