You're referring to a 51% attack. Changing the order of transaction can mean a lot depending on which transactions.  In an attack scenario it means that the attacker is trying to reverse his transaction.

Say Attacker (Alice) sends all the BTC  from Wallet A and buys something from Bill. And then sends another transaction for the same amount to another wallet she owns (Wallet A2). Bitcoin protocol considers that the first transaction is the valid transaction and all subsequent attempts to double spend this bitcoin to be invalid. In this scenario changing the order of transactions makes all the difference. The moneys are now sent to Wallet A2, Alice has the product from Bill and Bill was defrauded. He will not receive his BTC.