Channels determine how your traffic sources are grouped in your Google Analytics reports. It's important to define your channels so when you look at the Multi-channel reports your traffic is grouped the way you want.
Why is it mandatory?
A Channel in Google Analytics has rule-based definition. When you choose a channel, we use Google Analytics rule based definitions to populate appropriate mediums for you. That way, you or your team do not have to remember these rules. Plus, it'll help keep your nomenclature consistent and you can't make a mistake.
If you would like to disable Channel for your account or want to keep it optional, please contact us and we'll make it happen for you.