Beta is very important. You see, a simple spelling error can change one noun into a very different one in such a way that a spelling/grammar check will not catch it. It can transform one large inanimate object into something completely different and ruin not only the flow of the sentence, but the whole paragraph, chapter and entire fic, because it changes things dramatically.
Because 'how the plane crashed' and 'how the planet crashed' are two very, very different statements. In one situation, life for those not involved can go on. In the other, we all die a horrible (perhaps instantaneous) death and all life ceases to exist. Forever.
Oh, dear fic writer, our relationship was already off to a rocky start, but now I think all of your characters are dead so, sadly, the affair is over.