This is a service release which mainly introduces additional stronger DNS seeds as well as updated caching to improve sync times.
This is not a mandatory release however it is for very highly recommended for Smart Node operators as the improved networking should lower the chances of accruing PoSe Ban score.
Main differences between this and 1.2.15.3 are:
A) Significantly improved sync times
B) Significantly improved networking making PoSe accrual less likely!
C) Improved in client CN code handling.