Budgets are useful when you want multiple users to be able to spend from the same budget or potentially when one user needs to spend from different sources (e.g. cards, invoices reimbursements).
Examples:
1 user, multiple transactions on a card, specific purpose > create a virtual or project card
1 user, multiple transactions on a card, specific vendor > create a subscription card
1 user, single card transaction > create a single purchase card
1 user, multiple expenses (card transactions, invoices, oop) > create a budget
2+ users (working in the same team or project) > create a budget