I guess it's easier to get stuff out of the bag if it's beside you rather than having to bend down. This is even more important if the thing you want has got to the bottom of the bag. [Not a selfish view at all

]
Now while I like to think I'm an ultra-calm traveller that gets stressed about nothing, I do sometimes find myself doing checks every 30 seconds (it seems).
Bag, ticket, passport, phone, coat. Bag, ticket, passport, phone, coat. Having the bag in full view beside me helps this process.
It avoids the panic that can descend: Where's my bag?, Where's my bag? It's disappeared. Oh no, it's on the ground.