When a vital component fails, the options are to reach for a spare or go get another one. If you happen to live next-door to a DIY store, those two options are interchangeable. If your nearest store is thousands of miles away, then you’d better be organised at keeping spares on hand. But imagine having no store in easy reach plus not really having the room to carry spares.
That’s how it is if you’re lucky enough to live on the International Space Station. The equivalent of sending for a new part involves a multi-million dollar launch, but you can’t keep two of everything because physical space is at a premium.
Take this a step further and on to the proposed manned mission to Mars. The DIY store is now completely out of reach and the problem of how much you can carry with you is way more of a problem than it ever was on those short hops to the ISS or the moon.
Not that the spare parts problem is the only one or even the main one in the path of space exploration but it’s an issue, and it happens to be one that’s being tackled right now on the ISS where they’re experimenting with zero-gravity 3D printing i.e. making their own spare parts which is the only solution in the long run. If it works, it’s one more step towards further exploration of the universe. And if it doesn’t work, well ... it’s still one more step towards that same goal.
Every giant leap that humankind has made over the centuries has just been the final move in a series of tiny steps.