Imagine your traffic is like this, with a spike that is 2 times higher than usual average values (sometimes it can be x3 or x10):
In order to find the reason, of course you can manually go to Acquisition > All traffic > Channels, and then trying Referral, Direct, Social, (Other), Organic search to see if the spike is still there in one of them.
But this is not very handy.
Is there a clever tool in Google Analytics to guess the reason for a traffic spike?