Inefficiency - Banks and financial institutions spend billions on opex (ex. BoA spent $53 billion on SG&A and occupancy/equipment in 2020). Well written protocols and decentralized trust can increase efficiency and collapse this number
Permissioned access - For better or for worse, anyone can create financial products and a whole market for it without asking the SEC
Opacity - Instead of using paper/hidden digital ledgers, open source contracts lets one easily follow money flow through code and transactions
Access - Turns out the world is very big and lots of people are either locked out from traditional financial institutions or their preexisting ones just really suck.
(some examples taken from article https://parthchopra.substack.com/p/part-2-why-defi-is-not-a-...)