Why cause matters even if we live in a nondeterministic universe
First of all, and last of all: Scope. There will always be times when something has a higher amplitude or importance within a particular smaller scope but that does not outweigh the importance of things that might have to be postponed. In example: relationships. We build goodwill and strength into them, they are a joy battery, that is of course the most important over all. Build it strong enough, and it can withstand some space in which other things, like school or work, are more important. Feed it and we can make withdrawals against it in times of trouble. When things get really bad, I might lay my body down, like a bridge. But that is a final sacrifice and so reserved. Other things are not as great a sacrifice but become more important because we can do them ALL THE TIME.
There is a secret to that dichotomy. It is only a paradox if one looks away, perhaps by turning the other cheek. Jesus said that how you treat the least of His is how you treated Him. Think on that next time you react with disdain or apathy or god forbid anger at the needs of other Life forms.
So it doesn't matter if what I do matters because what I do matters at the very least to those I care about and I care ab out every Life form. I impart the meaning into my Life because I am paying atterntion to how my actions affect all the rest of the Universe. I am certainly wrong about some of it, but I try. And see, non-determinism is also a scope thing. Here, cause and effect seems to matter, even if it doesn't matter to the Universe as a whole. It worries about balance, but i can worry about Balance. Me doing good doesn't mean it will come back to me, but there will be more out there to splash on someone else. And isn't that enough?