How do cryptocurrencies help people who are unbanked or underbanked?

tl;dr: They don’t! (at present!). Period.

Take a long hard look at that photograph above. This is how majority of the world’s unbanked population looks like. Most people who keep advocating for the unbanked have never wandered 100 miles outside their general circle of comfort, let alone 10,000 miles out of it to live and interact with the truly unbanked.

Do you really think the unbanked are looking at systems that frees them from the clutches of the banking system.

They are not.

They (the unbanked) are barely able to hold their jobs together and survive. Their problems are mostly related to poverty, employment, food, water, shelter/housing, sanitation, basic healthcare and education.

HODLing and cutting the chains of banking is not even in their thought process, let alone something they would do in reality.

Cryptocurrencies (from a payments perspective - today, as of writing of this answer) is best suited for cross-border, where it represents a strong case. For domestic or regional payment systems, it presents an extremely weak case. At present, cryptocurrencies don’t help the unbanked in any manner that is significant of noting down.