How is Halo useful for zcash?

A rather broad question: how is Halo used in practice (or more specifically in zcash)? Is it in recursive composition, or just batching? If it is used in recursive composition, could someone give me an example?

An even broader question: If Halo is the answer, what is the question? PCD free from MNT-4/6 cycle?

Halo is of course ingenious, but I think I lack the big picture to fully understand it. At this stage any hint will be very helpful for me, thanks!

You can find more details on Halo here:

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