You should send a campaign if you would not like to include all recipients in a group conversation. Typically, campaigns are employed for lead and sales qualification or sales emails to multiple unique deals. 

For example – you add seven leads to Pipeline, and you’d like to set up an exploratory phone call. Or, you’re visiting Boston for the night and would like to invite all prospects and customers to a happy hour. There’s no need for everyone to see who’s on the recipient list. There is no ‘Reply-All’ for a campaign.