Linking a YouTube channel to an AdWords account lets you:
manage multiple channels from one Google account
use brand accounts to manage many YouTube channels at once or to manage multiple Google Accounts
manage one channel from one Google account
manage one channel from one Google account and let multiple people manage a channel
You can link more than one AdWords account to your YouTube channel and vice versa. You can’t link a YouTube channel to AdWords if a channel has no public uploads and less than 10 subscribers.
Read more here: https://support.google.com/youtube/answer/3063482