** Note: You must have Opportunity Teams enabled before establishing a default Opportunity Team. See this guide outlining how to enable and use Opportunity Teams.
Default Opportunity Teams allow you to set Salesforce user(s) to certain roles whenever you create opportunities. This can be helpful if an AE always works with the same SDR or Customer Success Manager, for example.
Note that you always have the ability to change the Opportunity Team after it is automatically created. See this short video explaining how default Opportunity Teams work and how to edit them.