By building a common protocol across the web people will be able to build and find platforms that suit them, and then can choose to (or choose not to) interact with people on other platforms through this shared language. If my server owner doesn't want to enable local search (as mine doesn't), then I'm out of luck and can't even search my own Mastodon posts (as I currently can't). A service like Meta can have a peek into my data for the purposes of showing it to users on Threads, but they wouldn't be able to follow me all across the web based on my data and usage habits since I wouldn't be using the Threads app.