I know that you can sniff packets from other computer in network by mitm attack using arpspoof, but why to bother if you can turn on monitor mode and sniff all trafic(if I understand correctly monitor mode lets you sniff all packets). Of course I have key to the network so I can decipher it.
You're correct, but only for wireless clients. If you want to trick wired clients into routing their packets to you instead of the real router, you'd want to arpspoof. This way allows you to capture both wired and wireless client's data.
Because monitor only allows you listen you in. You need arpspoof to invoke a response from a server or interfere with an existing communication channel