It's not entirely true that "const" is "just a comment," depending on the use case. In machines with super-limited RAM you can use const on globals to tell the compiler "put this in .rodata"

In other words, "const" (in a global context) can tell the compiler "you don't have to copy this to RAM, just read it directly from non-volatile storage." Obviously, that would be undesirable on a desktop computer, but if you're dealing with a wee little microcontroller, it's very helpful.