Before anyone gets too excited about this...
This is standard practice and more of a formality than anything else.
I've had to correct both liberals and conservatives on social media all day about this one lol (and gotten some rather aggressive hate mail in my DMs in the process haha... I was called a lib-"youknowtherestoftheword" by one conservative, and had two AOC fanboys from Cleveland threaten to come to my house haha -- let's just hope their moms don't give them enough allowance money for the ride-share)
The rules of the game:
Convention rules require roll call and nominations for any candidate that passed the delegate threshold, which included both Biden and Sanders.
...so I'm sure she was happy to do the honors given that she probably does prefer Sanders to Biden. But this wasn't her "taking some radical stance for what she truly believes in" (as some of her fans are proudly proclaiming), and it isn't some sign of "a fracturing of the democratic party" (that many conservatives are gleefully espousing)