Getting hung up on the transactional piece of this is tragic. Ukraine constitutes a society that has embraced (evidenced by their revolution, stiff resistance so far, etc.) enlightenment values - the worth of an individual, a rules based society, meritocracy, etc. Before we run to all the counterfactuals, these values are generally instilled in the culture (particularly the younger demographics). NATO essentially exists to protect this common, values-based order.
Not granting membership earlier was those countries (mentioned in other comments) over-rotating on transactional-based benefits, over value-based ones. That's why there's such frustration now over getting Ukraine into NATO now. Will it happen? No, diplomatically impossible while they're engaged in hot war. But the moral outrage is reasonable given the bloody price they've paid in defense of the values NATO exists to protect. To clearly connect the dots...considering Ukraine as part of NATO benefits NATO in that it allows them to uphold those values it exists to serve, giving it purpose and strength from the unity of its member countries (and moral investment of the individuals who serve its armies).