It has two purposes:
1- Assigns a value to MML internal variables. $0-$7=1-255
Example: "$0=64 V$0" instead of "V64"
2- Created a macro definition.
Example: "{MD=CDEFGAB<C} [{MD}]"
"Is it efficient?"- Probably, considering how little code it uses. Though, I would take out the "?".
"Could you make a compressed library out of it?"- Could you explain what the word "library" means in this context? I've heard of the term "library" before but I never fully understood it. *noob question*
You could use GCOPY.
GCOPY 0,0,0,400,240,0,240,0
GFILL 0,0,399,239,0
Also, just a warning, if you increase the size of the map above 27 the program becomes unstable. If you want a map that's bigger than that, I can help change it.