It matters because accounting has to ledger expenses and balance books. Its a super pain when an employee makes a non work expense and then maybe uses cash to pay off instead of the intracompany accounts, because then the outgoing and incoming doesn't zero out...usually needs a manual books adjustment to account for the discrepancies.

It wasn't your money to spend?

Accounting is a big problem.
And you can say it was jewelry for your girlfriend - but realistically it could have been literally anything else. I'm not saying illegal dugs - but it sounds really shady and it puts the responsibility on the company.
With prior communication it might had been okay with your boss, or he would have lent you his personal card for this.

It mattered because he could have fired you for cause on the spot, our entire legal/financial system is built on the assumption that you don't mix business money with personal money. Your company accountant(s) had to write a paragraph explaining to the auditors how you made an innocent misclick mistake and company policy is very clear that they don't do that, otherwise the auditors would have not signed your company's financial statements.

Bunch of people explaining why it matters legally and financially, your only takeaway is they're bootlickers. Yes, YTA.