The advantage is that the user does not need to split up files and directories into two or more (blank disc sized) pieces by hand.
assume every operating system can perform disc spanning on any media as a built in function; this is incorrect.
Some disc spanning schemes include a small program to reassemble the data set into the same structure it had on the source machine.
The use of disc spanning will in most cases make your files unreadable to the file-system.
Many users don't want to be bound to such solutions and use "Simple disc spanning" instead.