The SIL connector is for the prototyping area, should you need it. You'd solder it so as to bridge the two sub-areas and add a row of pins on either side to use jumper cables to route signals out to the ESP breakout.
Here's a different use of the prototyping space, designed to hook sensors with 3-pin landing:
... There's quite a lot going on on the underside but it is mostly a matter of minimizing the number of links needed.