I actually find it useful having two separate set top boxes, an AppleTV and a STB bundled with broadband (currently a BT TV Box Pro).

My BT TV package includes Sky entertainment channels through NowTV but it's massively cheaper to subscribe to Sky Sports directly with NowTV. That means using a second account because adding it to the original NowTV account would apparently have to be via BT at full price. Accordingly I have one STB with the NowTV app logged into the account with the Sport subscription, and the second STB with the NowTV app logged into the other account with the entertainment subscription.

Similarly I have a TNT Sports subscription as an extra on my EE mobile plan, and apparently that can't be added to the same BT account as my BT TV (although it is on the same BT ID confusingly) so a second STB is handy there too.

Perhaps my use case is unusual and other people will just pay more to have everything on one account (which is probably what BT/EE are hoping for).

I also find it quite helpful being able to periodically get a new box with a tuner for terrestrial TV bundled with broadband, as they do seem to break after a couple of years (unlike my plasma TV which is 17 years old hence the need for a tuner box).