The best product of Summer of Code is new contributors to open source projects.
An example from the Drupal community. The current maintainer for Drupal 8 Core, Angela Byron, was a student back in Summer of Code 2005, the very first year it was done.
Another example is the code for installing modules in Drupal 7 Core, which started as a Summer of Code project for Drupal 6.
There are other examples within this community of people who came and contributed and continued to contribute, or established their own consultancies, hence expanding the ecosystem.
Not every project goes to be a big thing, but the as a whole it is termendously useful.
Another benefit is that open source projects get some cash too from Google, in addition to cash going to the student.
Disclosure: I mentored for Summer of Code for several years. I got a T-shirt every year for the years I participated.