What is the purpose of a default judgement, and what can it grant?

The purpose of a default judgment in a divorce proceeding is to get a court judgment without the other party being there. It's very disfavored, because the judicial system is supposed to decide things on their merits, and that's not possible if one side doesn't show up and their interests aren't represented.

Your husband may not have realized what the Summon and Complaint meant and just ignored it, but once he figures it out, he'll want to un-do the default and participate in the process. Maybe the best way for you to proceed is to get his cooperation, and do the divorce as an uncontested proceeding. The guide linked below is written for non-lawyers and takes you through it step by step.

Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.