You have to define an obsolete path so that the workflow system knows which way to go when the step is made obsolete. At first glance this may seem unnecessary because most steps only have one outcome but it does allow a lot of flexibility.
It allows you to skip several successor steps in one go or define additional processing to take into account that a step has not been performed.
It also takes account of steps which have several outcomes. For example if a decision has two outcomes (approve/reject) you can let the obsolete path follow either one of the outcomes (e.g. accept).