The alternate ending of I Am Legend explained a lot more about the infected than the theatrical version, was much truer to Richard Matheson's 1954 novel, and made the rest of the movie much deeper. The alternate I Am Legend ending was first revealed on the Will Smith movie's DVD release, and quickly became viewed as the superior version. So popular was the alternate ending that the upcoming I Am Legend 2 won't follow the original, while Warner Bros. has made the I Am Legend alternate cut available to stream.
https://screenrant.com/i-am-legend-movie-alternate-ending-changes-explained/