Explanation
A decision split activity is a journey builder activity that allows branching the journey based on attribute values or engagement behavior of each contact. Contact data refers to data that is stored outside the journey, such as in data extensions or data views. Journey data refers to data that is captured at the moment of entry into the journey, such as from a Salesforce entry event. By using both contact and journey data in a decision split activity, NTO can compare attribute values from different sources using an attribute-to-attribute comparison. For example, they can compare the promo code from the transaction data extension (contact data) with the promo code from the entry event (journey data) and divert contacts who have redeemed the promo code before reminder emails are sent. References:
https://help.salesforce.com/articleView?id=sf.mc_jb_decision_split.htm&type=5
https://help.salesforce.com/articleView?id=sf.mc_jb_attribute_to_attribute_comparison.htm&type=5

The order ID field is conducive because it places all of the items that the customers want under one order. You can imagine how problematic it would be if the rules would be set in different areas. It will be harder for the company to track the orders of each customer.
This may lead to people receiving items that they did not order or not getting any of the things that they have ordered at all. The order ID field will make everything more organized and correct. The fewer mistakes that the company makes in processing orders, the happier the customers will be with their shopping experience.