The obvious answers are:
It makes for a better finish
It allows your own color scheme
It allows you to beef up the tail area and firewall without having to remove covering
It allows you to shim the front of the VTail to get the right incidence (and build the saddle up a bit)
But if time is an issue and all you are going to run is a TT40 or similar then just do the arf and be flying in 2 evenings. I have 3 arfs currently flying, and one of them is over 3 yrs old and raced a lot. Only work needed was at the firewall and shimming the tail.
All of that plus
Great Planes ARF covering jobs are pretty poor when it comes to longevity of the covering staying put.
An ARC will take one or two nights at most to cover then you can make the changes you wish to make, apply a unique color scheme and have a decent covering job.