Why does Emacs's source tar archive contain *.elc files?

[Maksim Kislyakov]

Good day,

 I was playing around compiling Emacs from sources. At some point, I 
have found out that '*.tar' files with Emacs sources contain '*.elc' files.

As far as I understand, 'elc' is byte-compiled version of 'el' file.

- Why does the tar archive contain compiled sources? That seems strange. 
Source code archive (which you use to compile program) contains compiled 
code.

- Also, what will happen if I apply some patch before compiling Emacs? 
'el' file's modification time will be newer that corresponding 'elc' 
file's modification time.