What is the variable substitution philosophy?

[Kelly Sumrall]

I'm working with v3.3.5 and using some examples found on the net. Some of my variables aren't getting substituted, specifically my bracketed variables [$var$]. Looking into the code, I noticed that it skips over quoted and bracketed code, leaving the variable name intact ($var$). Reviewing past commits, it looks like it used to do a blanket RegEx variable replace in the entire script. Why did this change? What problem is fixed by not replacing the quoted and bracketed variables?