If you *will* be connected to the internet you *will* need to install security updates; Internet is not the only source for packages (&updates). If someone installs Debian on 3 computers (or repeats installation 3 times), it shouldn't mean to use 3x internet traffic repeating installer downloads.
A) This user can save and reuse packages cache and apply updates at the convenient moment. B) When repeating install at the same computer (e.g. changing install decisions such as partitioning, architecture, etc.) the user only needs updates on final one. Most of internet uses on the world have really low bandwidth accessing to the internet, and making (some) unnecessary downloads can be a money & time problem. + I've participated on an "install party" where only half of Debian installs could be done because of this issue.