To make it look old, or well used....
There is some speculation that it may inhibit rust.... I don't know that to be true, but it has some logic behind it...

I do think that developing a natural patina is preferable to "faking" it.... Is kind of like pre-washing /tearing / and beating up jeans and selling them for more money that new ones.... instead of actually wearing them comfortable, you can buy them already worn out....

I don't think it worth my time... but, to each his own...

Doc