In high society, wearing a tiara isn't an indication of royal status. Instead, it's a statement that you're married, and therefore shouldn't be approached by a gentleman. Presumably, wedding rings aren't a big enough sign. You'll note from photographs that, prior to their respective weddings, neither the Duchess of Cambridge nor the Duchess of Sussex wore tiaras. In all future photos, you'll never see them without one when the occasion calls for it.
We assume unmarried women at these events spend the evening with nothing on their heads, fending off male suitors with a stick! Old traditions dictate that any woman whose head is completely exposed at such an event is declaring that she's 'on the market', and therefore open to propositions. Woe betide any young woman who turns up at one of these evenings without being briefed on the conventions!