I'm not sure the previous poster is 100% correct but honestly I have not looked into it in depth. I know the times I have gone through it the journals did provide typesetting and copy-editing for some aspects of the paper.
However, to get to your actual question a big reason people use journals is simply the name. As a researcher getting your paper published in Nature is not only big because the prestige but presumably more people read/see nature articles so you have a better chance at high impact.
However, to get to your actual question a big reason people use journals is simply the name. As a researcher getting your paper published in Nature is not only big because the prestige but presumably more people read/see nature articles so you have a better chance at high impact.
TL;DR Their value is their reputation and reach.