Rashi is explaining from where the idea of Bedika originates. It is because of Bal Yera’eh. In practice, once we are Mevatel there is no need to do a Bedika Mede’oraysa, and it is Mederabanan that we are Bodek anyhow.
Tosafos also explains that Bedika is necessary because of the Chomer of Pesach that we see in the Torah.