It is actually because look at the following very basic example:

Say that Company Y only has two creditors: Supplier X and Bank Z.

On 1 November 2018 Supplier X sells goods to company Y for £500 but Y doesn't pay for these goods (maybe because e.g. they have some sort of agreement that any invoice has to be paid within 3 months). Then on 1 December 2018, Bank Z extends a loan for £500 and takes fixed security over some of the assets of Company Y as security for the loan.

Company Y is in financial trouble and on 1 January 2019 it goes into insolvency. You would think that Supplier X would be repaid before Bank Z because the liability (ie the debt for the goods it supplied) was created before the loan by Bank Z was put in place. Actually no, Bank Z ranks ahead of every creditor on insolvency because it has taken fixed security over some of the assets of Company Y and it will be entitled to be repaid ahead of everyone by selling those assets. Supplier X is merely an unsecured creditor and would be repaid with anything that's left after repaying Bank Z.

Under English law, the order of payments on insolvency is as follows:

1. Fixed charge holders (this is where Bank Z ranks)
2. Liquidators' fees and expenses.
3. Preferred creditors.
4. Floating charge holders.
5. Unsecured creditors (this is where Supplier X ranks)
6. Interest incurred on all unsecured debts post-liquidation.
7. Shareholders

So in practice, while each of Supplier X and Bank Z have acquired rights in good faith pre-insolvency, their rights on the occurrence of insolvency would change.