There are several reasons I wanted the source code.
As noted in Discover and display Aave debt tokens - #3 by rptb1 the Portfolio dapp doesn’t allow me to add an Aave debt token. In the Knowledge Base I found text saying that the Portfolio dapp has its own internal list of allowed tokens. Since the dapp is closed source, this list is effectively secret. I was interested in modifying the dapp to show Aave debt correctly and possibly submitting this back to the source.
(And of course all the usual reasons I want anything handling my money to be open source: auditing, public scrutiny, etc. etc.)