You can read the materialized view while it is refreshing, you just get the old version of the data. You could do that by doing delete and insert as select in a transaction, but that's not practical for tens or hundreds of thousands of rows.

On Oracle, this is also useful because it works kind of like creating and dropping partition, instead of doing delete then insert, and that's better for the DB because of how Oracle deals with tablespace and blocks (and Oracle's truncate is non-transactional, I think), but that doesn't do much on SQL Server vs. postgres discussion :)