I read a lot on the web about this topic, but the answer I got mainly is "depends on how composer will write the sheet music", and it seems a convention.
What (in practical) will change? Because if that's the same, there is no need to use the "bottom" number at all (since its power of 2, and since /2, /4, /8 o /16 seems to do the same).
Really don't get why a composer will write 4/8 instead of 4/4. Any tips, suggestions and help to cover this and get some reasons?