Top-level command invokations not compiled

Hi,

There are many places where the following note is mentioned:

"Top-level invokations of commands are not byte-compiled, so be aware to put
most of them into procedure(s), so they run faster." -...or something like
that.

Well, why aren't they compiled? What is the reason to aviod compiling them?

--
Pozdrawiam! (Regards!)
Googie
.