[lltx] why user-defined callbacks?

[Arno Trautmann]

Hi, me again,

I forgot to ask that question in the last mail: What is the advantage of 
a user-defined callback (still talking about luatexbase-mcb) over a 
user-defined function? Maybe I still don't get the idea of a callback, 
but in that case I don't see the difference. Is there any and if so, which?

cheers
Arno