Because if you know who's supposed to be in quarantine you can take steps to verify that they actually are. And also because people are simply less likely to comply with rules when they know no one can tell if they're following them.
> I don't understand why you think an app that can promise to keep people's personal data private would gain lower adoption than one that was explicitly collecting such data. Maybe I've misunderstood what you're trying to say here but it would make more sense to me that an app that can preserve privacy would have higher adoption, higher trust and therefore higher compliance levels.
Again, because you're comparing "opt-in and private" with "opt-in and non-private". I'm comparing "opt-in and private" with "mandatory and shares user data with the health authorities" (which is different from "[opt-in|mandatory] and public" in that in the latter case any member of the public can get information about infections; I think South Korea are/were running a system with that level of sharing).
I'm already disappointed that the UK government feel it necessary to keep track of personal data (they announced recently they were not going to use Google/Apple's API for this reason). I really hope they don't try to force people to use the app or follow its directives.
Sorry but I don't understand how keeping contact information private or public has any effect on the compliance of people to stay in quarantine?
> So, my — non-expert — conclusion at this point is that privacy-preserving digital contact tracing probably isn't going to be that useful because people either won't use it or won't follow the advice when doing so is inconvenient (e.g. because it puts their employment at risk). But digital contact tracing in general seems like it can help, if you can drive both usage and compliance high enough.
I don't understand why you think an app that can promise to keep people's personal data private would gain lower adoption than one that was explicitly collecting such data. Maybe I've misunderstood what you're trying to say here but it would make more sense to me that an app that can preserve privacy would have higher adoption, higher trust and therefore higher compliance levels.