If the A-10 Warthogs are so useless to the USAF inventory, why are they being sent to Israel as an immediate action airplane?

The A-10s aren't "useless" to the USAF inventory - they are being replaced primarily because they are viewed as a single-role airframe (CAS) and would likely not survive if facing an adversary with modern air defense capabilities and if we didn't have at least a favorable air situation at the time of their employment.

Yes, we (the Army) love them. They became the darlings of the Gulf War, racking up thousands of vehicle kills to include about 1/3 of all tank kills during the conflict. Anyone that has been on the ground when they come in can attest to how close they can deliver accurate CAS to engaged forces - they are basically a slow-flying ordnance factory that we view as being able to chew up whatever is on the ground with the 30mm cannon or by delivering from one of the 11 hard-points carrying the heavy duty stuff if the 30mm doesn't do the job.

On the other side of the argument is that they ARE great - as long as there is no serious threat to them. They are durable, but unlike the popular myth we ground-pounders have about them, they are not invincible. We lost four of them during the Gulf war to 1980s era air defense systems and MANPADS and some others (3?) were taken out of operation because they were too damaged to be airworthy. Even when they were first put into operations (~45 years ago), planners expected to lose every A-10 in the inventory (>700) within a few weeks of combat with the Soviets if they invaded.

However, getting back to the specific question you posed about "why are they being sent to Israel as an immediate action airplane?". It still remains the pre-eminent precision CAS platform we have in our inventory ... as long as there isn't a serious anti-aircraft threat. The anti-aircraft capabilities that are present in Gaza have been very limited so far based on what's been used against the Israeli aircraft. To date the only threat has come from old SA-7s MANPADS although they claim to have more advanced MANPADS such as the SA-18 and SA-24 (again, no evidence of that and only SA-7s have been used against Israeli F-16s).

Note - this isn't new to the region as a squadron of A-10s were deployed earlier this year* at AFCENT's request due to increased tensions from Iranian-backed forces in Syria.
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* https://www.airandspaceforces.com/a-10s-centcom-bolster-air-force-presence/

Its unfortunate that AF brass always looked down their noses at the A10. And if you ask any grunt from the marines or Army they have always LOVED

They are not sexy thus they must be scrapped.