Plus a tank bag might offer a little protection should you get thrown forward in a shunt, protecting you from the business end of the bike (Clocks / Handlebars etc)?
Also perhaps offers more security on the tank, as nobody is going to try and rip that off, infront of you.
At one point in time the MSF taught about the 'rider triangle'.... the term was used to describe the distribution of mass, with respect to the contact patches of the bike.
If the patches are the lower two points of the triangle, then the highest spot should be somewhere between those two points, to keep it stable and balanced. The rider's head is likely to be the highest point 'above' the contact patches, which would lead you to think that's the 'point' of the triangle..... it isn't, really.
The center of mass from the combination of the rider, the bike and whatever else is stuck on the bike will determine where the 'point' actually lies. Anything that sticks further away from the centralized ideal point, has more leverage against the center, than something closer in. This is why motorcycle design has gone to more and more compact, dense designs.... compact means faster rotation.
This can be applied to anything we add to the bike...... if you want to wheelie, put the mass higher and further back. A passenger on a sport bike has a huge effect on handling, due to how far 'up' and 'back' that moves the center of mass.
Luggage on a bike can affect the triangle by the same process.
A back pack is 'high' compared to the center of mass.... best for anything in there to be as light as possible.
A tail bag is 'back'-- just like the passenger, their mass is almost directly over the back wheel.... you can put more weight in a tail bag, because it will always be lower than the center of mass of the rider, so the impact is less -- the lever is 'shorter'.
A tank bag? It's closer to the center of mass than the rider is. Effectively inside the triangle. If anything just a bit 'forward' of the normal center of mass. Most likely to have the least impact on the handling as a result of the change in center of gravity.
An old stiff fat guy like me isn't going to be resting my chin on the tank. The area that is 'filled in' by my daily tank bag (soft sides, maybe 6" in height) doesn't interfere with my normal operation of the bike. Even when I fill the 'daily' bag full, is only grows to around 10" in height, still easily worked around. -- If I fit the two sections together and the bag then gets to be 18-20" tall, that's an issue. I can still reach the controls and operate them without difficulty -- but I can't see the speedo.
I generally just strap the big part of the bag on the back seat, then.