The swapping of sections means that there is more variation in gametes. The same happens in the other parent.
Therefore if the same two humans reproduce again, the gametes that fuse will not be the same as the ones that fused to make the first child, you see?
So imagine that you had one kid with your partner, and your gametes didn't switch DNA sections during meiosis. All of your gametes would have the same combination of DNA, or at most, two different combinations.
So let's say you decide to have another kid with the same partner. Without proper meiosis, the kid would be identical to the first, whereas if you had proper meiosis, it would be different.
Because of the difference, in the harsh world out there, if the first kid wasn't fit enough, at least the second one would survive. If both were the same, neither would survive.
TL;DR- the way meiosis occurs means there is more variation in gametes, which means more variation in offspring.
More variation in offspring= better chance of some of your offspring surviving = better chance of the race surviving due to variation.
Sorry if I ended up repeating or paraphrasing some of the things I said before.