Bleaching goes to completion and has a wider effective temperature range, and so technically a starter only refines the end result for the photofinisher who must get good results in the standard process. If by some chance, you observe a result that is off due to bleach or fix, you can repeat those steps to repair any problem that lack of starter might, on a long shot, cause. The photofinsher cannot.

The starter is a "just in case" chemical. It is not mandatory. Developer starter is the only mandatory starter that must be used with a replenisher.

I think I have answered this same question in 3 threads just today.