Erik Darling doesn’t update all the rows at once like some barbarian:
The goal of batching modifications is to accomplish large numbers of row modifications with exacerbating locking problems, and being kinder to your server’s transaction log.
There is generally an appreciable difference in transaction time in modifying 1,000 rows and modifying 10,000,000 rows. Go try it, if you don’t believe me.
Click through for more information on good ways to set up batching, including the use of the highly underrated OUTPUT ... INTO ... clause on DML statements.