Selecting the Re-evaluate Workflow Rules After Field Change checkbox
on the Field Update Edit page allows you to set a workflow field update
to re-evaluate all workflow rules on the object if the field update
results in a change to the value of the field.

• If the field update changes the field’s value, all
  workflow rules on the associated object are re-evaluated. Any workflow
  rules whose criteria are met as a result of the field update will be
  triggered.
• If any of the triggered workflow rules result in
  another field update that’s also enabled for workflow rule re-evaluation,
  a domino effect occurs, and more workflow rules can be re-evaluated as a
  result of the newly-triggered field update. This cascade of workflow rule
  re-evaluation and triggering can happen up to five times after the initial
  field update that started it.
• Make sure that your workflow rules aren’t set up to
  create recursive loops. For example, if a field update for Rule1 triggers
  Rule2, and a field update for Rule2 triggers Rule1, the recursive triggers
  may cause your organization to exceed its limit for workflow time triggers
  per hour.
• In a batch update, workflow is only retriggered on the
  entities where there is a change.
• Only workflow rules on the same object as the initial
  field update will be re-evaluated and triggered.
• Only workflow rules that didn’t fire before will be
  retriggered.
• Cross-object workflow rules aren’t candidates for
  re-evaluation.
• Cross-object field updates that cause a field value to
  change don’t trigger workflow rule re-evaluation on the associated object.
• An approval process can specify a field update action
  that re-evaluates workflow rules for the updated object. If, however, the
  re-evaluated workflow rules include a cross-object field update, those
  cross-object field updates will be ignored.