Why I should delete index form the filed if there is secondary index field?

I reflect the "_layouts/15/EditIndex.aspx" to see how we can delete the SharePoint List indexes, and I found the following:

  protected void DeleteIndex()
        {
            if (this.m_indexId != Guid.Empty)
            {
                if (this.m_secondaryField != Guid.Empty)
                {
                    this.m_list.FieldIndexes.Delete(this.m_indexId);
                }
                else
                {
                    SPField field = this.m_list.Fields[this.m_primaryField];
                    field.Indexed = false;
                    field.Update();
                }
            }
        }

I'm wondering why they check if there is secondary field and if not they delete the index from the filed. thy can delete the index directly (FieldIndexes.Delete(this.m_indexId);), and no need to if statement.

 protected void DeleteIndex()
            {
                if (this.m_indexId != Guid.Empty)
                {                       
                        this.m_list.FieldIndexes.Delete(this.m_indexId);

                }
            }