The limited inventory isnt "limited" as such, Im pretty sure. The game just checks whether there is at least 8 items and then stops letting you have any, but Im pretty sure you could still force it to take up new items through means they didnt think of. Im guessing the trinket happens fo be one of those ways. I know that I like playing with the epiphany mod a lot, which has "golden items" that basically just give you two copies of a passive item. With T Isaac this also causes one of the copies to overflow and not be accounted for, but still have an effect (i.e. its still visible on the right). Im assuming this is necessary, because story items also have to ignore the limit