Your question seems to assume that things can't have value if they're temporary. But we care what happens today even though today will end in a few hours. This is because one important source of value is what we experience: it matters to us to avoid pain, to enjoy the lives we live. That's a good reason to find out about life even though life won't last forever. It may also be that some things gain in value by being temporary: we might not find rainbows so beautiful if they were in the sky permanently.