Assembler's a great illustration of this. Assembler imposes very few constraints on what you can do. Which, in turn, makes assembly language programs very difficult to manipulate, because there are just so many special considerations to worry about.

A language that doesn't let you decide what registers will be used to store data, or interleave conditional branch statements in clever ways, or carefully select which instructions you use in order to tightly synchronize the program's execution to some outside signal such as the monitor's vertical retrace[1], is a language with a lot more freedom to do things like compiler optimizations.

[1] Dated example, I realize. Unless you count playing TIS-100, it's been a minute or two since I last did anything interesting in assembler.