Just in case.

[Upt' North]

Just in case it falls off.
Just in case you haven't got a clue where to put 6ft of shoulder strap, in metric that's right around 6ft.
Just in case you're proper bored.
Just in case you can't help yourself.
Having bought a Kappa tank bag the security/shoulder strap just wasn't going to work out, there's a separate handle anyhow.
But not having had a bag with a tank ring before I thought I should retain a strap......you're good at guessing.......just in case.
I'm assuming similar bags will have similar stupid attachments. This was my remedy.
Upt.

 

[Obo]

Snap around the bar, check.
Ring around the lines? Not so sure. Oh wait, that's the holder on the bike isn't it.

 

[Sadlsor]

That's good.
No one wants to slip off while riding.

 

[dduelin]

Belt and suspenders is good but I have to say without a tether I never yet lost a tank bag from falling off when riding.