It is putting the significance in the foreground. An if statement is a semantic part of the language. Highlighting that is calling out that there is a higher level of programming going on there than a sequential list of instructions. No programmer needs the keyword “if” to be highlighted to know what it means.

Also, how does having strings highlighted put significance in the foreground? The only thing it will show you is if you missed a delimiter. It doesn’t call out parts of the program that do something noteworthy.