How is this advantageous over str.format?

It does often perform better to make a prepared statement and then call it several times with different values of the bind parameters. But just as often, the difference is negligible. The query may be a one-off in the lifetime of the script. But anyway, if you have a t-string based interface

t'select name from mytable where id = {myid}'

then the SQL library is free to transform that into a prepared statement which it can execute hygienically and even cache for future calls of the same t-string.