There are two general areas:

• The history of the internet is full of examples of companies taking data about or creative output from people and trying to make money from it without permission, in ways the original creator might not like. Nobody has gone there with a Fediverse scraper or search project that we know of yet, but it's going to happen if the Fediverse gets big enough.
• Some people want to be able to easily share things with a certain audience without them being easily discoverable by a different audience. There are of course privacy settings to control visibility and software like Matrix that provides not only access control by cryptographic security, but those add friction. It's only possible for this group to have it both ways if nobody develops good search tools, which turns some of them into bullies.