1. The DBI module can compare the number of placeholders to the size of the array, and tell you if there's a mismatch (rather than waiting for the database to tell you. So it's less work on the database, and more independent).
2. Without placeholders, the database is being passed a different (lexically) statement every time, and has to parse it each time (which is overhead). The place holders version only needs preparing once, and then just gets the appropriate values filled in (which is less database work).

--
Tommy
Too stupid to live.
Too stubborn to die.

See Placeholders_and_Bind_Values and quote in the DBI docs.

-jehuni