Our old default was 14px, now it’s 15px. All text will look slightly larger.
Why?
There’s some evidence that larger font sizes increase readability, and larger font sizes are generally becoming more common across the web (you can easily find some sites that are making body text as large as 20px!). So this is a minor bump in that direction.
One thing not mentioned, but should have been: this is yet another normalization between mobile layout and desktop. Mobile has been at 15px default for a while now.