Glad you asked!
The API traces possible situations where the virus could be transmitted. It does not trace contacts at a level which would be useful in tracking the transmission chains backwards or forward. The only thing it does is that it enables warnings for possible exposure. Interpreting those alerts is extremely difficult, as they do not give any information on the exposure situation (indoors, outdoors, masks on, etc.)
If the application tracked the exposures with all the available technical data (from whom, where, when, exposure time, signal strength), it would become relatively simple to calculate transmission probabilities in different situations. There could then be forward warnings in the transmission chain, and many now unknown transmission sources would become clear. There would also be immediate warnings of superspreading, which is an important mode of transmission in this epidemic.
Also, this data would give a lot of very valuable information on how the virus spreads in population. That information would help to plan the most effective countermeasures with the minimal side effects. Here we talk about billions in whatever currency unit you prefer.
So, an effective tracker would save lives and economies. With the tiny little downside that this level of tracking would throw privacy out of window.