* Inspiration - the space program was a huge morale booster and directly responsible for a lot of people going into the science/engineering professions.
* Dual-use technologies - most of the problems you end up solving have other uses, from little things like velcro up to ICBM and other weapons tech. Or, you could think of it as giving an alternative to those who want to work on big explody things, but aren't comfortable with them being used to kill people.
* Economic stimulus - those billions of dollars aren't just disappearing. A large amount of the space industry is US based, and that money goes to them and their employees and creates/maintains both skilled and unskilled jobs.
* Scientific research - There are plenty of things we don't know that would benefit from modern sensing or analysis capabilities that weren't available in '72.
* Human progress - At the rate humanity is expanding, eventually we're probably need to gather resources off-planet, whether it's orbiting solar power satellites or asteroid mining, that tech needs to be developed. The longer term goal of actually colonising some place else is also pretty important if we consider progress & survival on a long enough scale.
All of these are I think good reasons why it should be done (again). That's not to say it should necessarily be done by the government, but it's certainly not money wasted in the way people often claim.