Just knowing something is true without knowing why is a lot less helpful in figuring out what else is true.

You are right that computer generated proofs are often tough to understand, and that understanding is often the intended goal of writing a proof. But proofs can be useful for reasons besides human understanding, especially so in engineering. For example, when a browser establishes a secure session with an encryption protocol, the reason it is secure has to do with the difficulty of finding a proof that eg certain numbers factor, but no human cares about this proof. As another example, a compiler can use a proof that a program is type correct to optimize it, but humans would not find such proofs interesting at all.

That's great, but not relevant to mathematicians doing their jobs.

Knowing which conjectures were true would probably help mathematicians work more efficiently. At the very least nobody would spend time searching for counterexamples to true conjectures.

Existence proofs remain popular however, as do establishing upper or lower bounds.