Instant yeast tends to be very aggressive/successful so what happens if you use a small amount of instant yeast is you get a slow rise but it's a monoculture microbially speaking versus if you use a typical sourdough starter (called "madre" in Italian baking) there are several strains of bacteria which are about equivalent in terms of how fast they multiply. So the final dough has a more diverse microbiome. The theory is that leads to a more complex taste because of the way the different yeast strains digest the sugars etc and affect the taste of the resulting product.