What is the advantage of using starter cultures for growing bacteria?
Growth can be quite slow for some species under certain conditions when the concentration of cells is too low. Log-phase growth is powerful, and so one would like to keep cells in this state for the experiment at hand. Different genes are expressed then compared to a stationary phase.

In addition, you'd like your culture to out-compete a contaminant if there is one. That is more easily accomplished with a starter culture, which is then used to inoculate a larger culture for scale-up. Inoculating directing into the large-sized flask may allow your bacteria to enter a stationary phase, thus giving an opportunity for other species to out-compete your bacteria.