i’ve wondered this myself a few times, Roofus, but the doctrine of grace, that allows for sin covered by Christ’s blood, seems to negate this judgement, which seems wrong.
so the idea that i’d been brought up with about all my sin being forgiven already, so i’d just go straight to Heaven, seems to contradict a few verses like this one. there was talk of a “rewards” judgement for Christians only, but that was i think a poor way to get this aspect of Scripture to agree with the once saved, always saved my Calvminian tradition raised me in.
this is my very poor take on this topic. i believe that the reason this life is important is not just for us to prove ourselves, but for us to grow up slightly away from God, given the choice to move closer to Him or further away. i think that this experience, once all is fulfilled, will drive us even closer to Him than if we’d simply grown up in Heaven.
I think God allowed sin to occur, and evil to pollute creation so that He could be the one to redeem it, and once His bride is cleansed of the muck, she will never doubt the love of her Husband ever again. she did once, in a sinless state, in the garden of Eden.
now i think that given what we know, relative to each of us, we need to do the right things with what we’re given, and we’ll be judged for that. but don’t forget that Paul says (i forget the reference) that we’ll be judged, and the works that are good will survive, and the works that were bad will not. and if all our works are bad, then we’ll suffer loss but we ourselves will be saved, but though as by fire
if you can think of it this way, if i am in heaven, where it’s EASY not to sin, and i do good, what reward is there for me? but if i on earth, where it’s hard to do right, do a good thing, then God will reward me. if i go against my conscience (as i have many times) or work in the strength of my flesh rather than in God’s strength (again many times), then my works will be rubbish and i will suffer loss for that. but His grace is sufficient; His word does not return void; He does not cast off forever; and where sin abounds, there grace abounds all the more. so i can be confident that flawed and wretched as i am in this life, the judgement will NOT be the end for me. i will be washed clean, and be the wiser for it, and the more grateful.
this to me is why this life on earth is so important.
but i will say that not all of us get a real life on earth. babies die, lives are cut short. to God then, this life is not always so important as that, as He will not neglect to save those lives too.
if i were to summarise, i’d say it’s because this life is school…Heaven is the Real World that follows. in school you aren’t given all the answers, but you’re tested and judged. eventually you graduate, and enter the Real World. note: this analogy does not allow for expelling as that’s a whole other aspect of the debate 
sorry, that was long and involved, but hopefully was more or less what you were after