Why does the frequently occurring delay between the time an offer of employment is accepted and the individual starts work exists?

Question $1$ options:

Time is required to check references and accomplish a pre$-$employment physical examination.

A legally required orientation must be accomplished first.

It is necessary for the human resources department to clearly remain in control of the process.

A certain delay is allowed in case the incoming employee has to work out a period of notice e at another job.