The point is that if you're in a asymmetric position where you can't do much damage directly, then whatever you can do to make the other side waste expensive resources while putting them on constant alert is a win.

You don't need a warhead, or a viable rocket, to do that. You need something that looks enough like a viable rocket to force a response, because the other side knows that x% are real.

If that thing is cheap enough for you to fire large numbers of them, you multiply the problem for your opponent. Cheap enough, and you have the potential to overwhelm the capacity of their countermeasures entirely, at which point you increase the chance that some of their real rockets will make it through.

Also, because it costs a lot and there are only two benefits of ballistic over cruise (if you exclude delivering nuclear payloads, which Ukraine doesn't have): it's very fast and hard to intercept. Both are needed sometimes, but often not a requirement.

Ukraine is comparatively small, so air defenses can be packed close, Russia is big and harder to cover with air defense systems, so drones and cruise missiles are a better investment for Ukraine, since they can overpower the AD locally and are much-much easier and cheaper to produce, meanwhile ballistic is a better investment for Russia, since anti-ballistic systems are even harder to build and cost a lot.

I used to think the US dollars were well spent, because we felt it was morally important to deliver precision strikes which had higher cost requirements. Recent evidence demonstrates that is insufficient when the wetware making the targeting decisions is faulty.