How is Network Difficulty Chosen?

Every 2016 blocks (~2 weeks of blocks) the average for those blocks is calculated and the difficulty is adjusted to bring block times back to 10 minutes.

Why 10 minutes block time is important?

10 minutes is just long enough for all the nodes on the network to pass the hashed block around the world. 

Again the whole point of PoW is to make sure there is 10 minutes time delay after the previous block added to the chain, so the new potential blocks would have time to prorogate the network and the final winner (the one that is accepted by the most nodes) emerges.   

https://braiins.com/blog/bitcoin-mining-analogy-beginners-guide