TL; DR: Being contagious in a pandemic has consequences for everyone near you. That includes people you know, and people you don't.
In some contexts (like universities and workspaces), if a certain number of people monitor their current health status and symptoms, after adding information about the communities they belong to, and the locations they frequent, we can institute a bottom-up version of contact-tracing.
So far, social distancing has usually started at the point when cases are already present in the community.
This is always too late, given the early asymptomatic nature of COVID-19. People who have already contracted the virus are going into quarantine, possibly with their family. Another relevant factor is the different effect the virus has on people, especially by age.
If three of your friends at university become symptomatic a week after entering quarantine, you need to know that. It has very different consequences for grandma at home. You might not even be showing symptoms yet, but be shedding the virus.
We're asking for there to exist a gestalt intelligence: one that can find contagion among a group of people who've at some point interacted, whether as friends, or by being at the same places, or belonging to the same communities
Once you begin showing symptoms, and reach a certain level of discomfort, you make your way to be tested.
Contact tracing begins when you test presumptive positive at a clinic
But we know that COVID-19 spreads asymptomatically. And that the virus sheds the most when early symptoms appear.
If you begin to have mild symptoms by yourself, that's one thing.
But if you also know that people at school/work are also beginning to show symptoms, that's a confirmation that you are in a contagion.
When people begin to show symptoms, their close friends might know about it, but not other people who use the same locations, or belong to the same communities.