For some of you DCS on Steam may not offer any advantages, but for some of us it does. For example, I live in Thailand and would be lucky to go get download rate of 200KB/s from ED's file server, normally it just swing around 30-100KB/s. While on Steam I can get a download rate upto 2.4MB/s which save me lot of time.
Also, Steam automatically keep all the modules you own up to date, so you don't to download separate installer for each which make thing much simpler, especially when you need to reinstall your PC system, just one click and Steam will download and install DCS and all modules you own for you.