filegroup

What is the advantage of having two secondary files inside a filegroup.
For example, I can create a table and place it inside a filegroup which obviously will use the secondary files.
Not sure why some DBAs create just one but otherscreate more than one secondar files in a filegroup.
I have read the booksonline but can not figure out the advantages.
Thanks

suppose you have a file in filegroup on a hard disk and the hard disk will be full by your file now you can add another hard disk and add a file in filegroup and place it on new Hard disk