There are several reasons why people would hold multiple Biz Plat cards but the most common reason I see is because they are able to receive hefty sign up bonuses due to readily and easily available NLL applications. So they justify holding multiple cards with $895 AFs each because the value of the SUB is a lot higher than that - right now it's as high as 300k, but even for 200k-250k it's worth it. After the first year they can no longer justify holding the card and will most likely cancel unless they get a good retention bonus which as of late has been few and far between. But they are totally fine with this as they continue to cycle through new cards every 3 months and thus continue to hold multiple of this card.

These people will use whatever benefits come their way easily - they'll combine 2-3 Dell benefit credits to get themselves a new laptop or Bose headphones (or whatever strikes their fancy from Dell website) and will use the airline fee credit for themselves, friends, family, and employees for checked baggage, lounge access, seat selection fees, etc. FHR, Clear, and Global Entry are easy credits to use as well for themselves or anyone else. Plus there's the lounge access which has value if they use it. The AF is easily covered by using the above benefits, but like you said they may not even care about all the benefits because the initial justification for getting the card was the high SUB. Usage of all these benefits are just extra credit.

I'm sure there are people who hold multiple Biz Plats for other reasons (i.e. separating expenses for various businesses) and don't care about squeezing the juice out of every credit but I find that that isn't so common precisely because the AF is so high and for all those other reasons there are better cards out there.