Why are carabiners weaker with their gate not locked?

Is the idea that the gate could open and what's in it can come out? Does locking it actually provide physical breaking strength somehow?

It’s an open gate that weakens a carabiner. An unlocked, but closed carabiner is full strength.

Carabiners are stronger when the gate is closed. Indeed the rope can escape from a carabiner in some wily ways. Locking encourages the gate to stay closed. Locking does not generally increase the strength of the carabiner (beyond than keeping the gate closed). 

In fact many lockers are WEAKER than a typical non-locking biner's closed-gate rating. It's the larger, HMS size lockers that tend to be weak*. The reason is that in a big biner the rope (or test pin in the case of testing) sits farther away from the spine of the biner, and biners are strongest when the load is along the spine.

* Smaller lockers are typically the same as their non-locking kin e.g. Petzl Spirit is 23 kN major axis in both locking and non versions.

Yes, it actually provides physical breaking strength.  They stretch enough that the gate/slot or keylock tightens up and takes load.  "Open" means that interface doesn't happen and the gate doesn't take load.  It only has to be open a little bit and only for an instant (if that is the instant that the load comes on).

Jay Anderson wrote:

Yes, it actually provides physical breaking strength.  They stretch enough that the gate/slot or keylock tightens up and takes load.  "Open" means that interface doesn't happen and the gate doesn't take load.  It only has to be open a little bit and only for an instant (if that is the instant that the load comes on).

I thought the "Interface" was between my skin & the rock ?