Consider it this way:
A normal person uses a commlink and 'trodes or a datajack. They get a Matrix persona with a small amount of Data Processing and Firewall, and no Attack or Sleaze. They can do legal stuff, but nothing else; and they can resist Matrix attacks, but not very well. (Or they can just use its screen, which I don't think counts as "Matrix" access at all (they have no persona etc.))
A decker uses a cyberjack and a cyberdeck. They get a Matrix persona with decent ASDF stats. They can do illegal stuff as they have Attack and Sleaze stats. Cyberdecks include everything a commlink has and don't then need a commlink to get online (this was clear in 5e and doesn't seem to be explicit in 6e, but I think it's change blindness and not an intentional change.)
Deckers might choose to also carry a spare commlink, perhaps for subtlety reasons (cyberdecks are illegal while cyberjacks require licences [1], and decks are physically large enough to be noticeable) or as a backup. I have no idea what happens if a decker goes online with a cyberjack+commlink combo; can anyone shed any more light on that? I'd guess the cyberjack's D/F stats override the commlink's, but I can't find a specific rule.
[1] Which, thinking about it, is a bit weird, no?