Why should a team combine end to end tests with unit tests rather than relying on just one type?

Unit tests are fast and pinpoint exactly which small piece of code broke, but cannot catch problems that only appear when pieces are combined together, like a broken navigation flow. End to end tests catch these bigger, real world problems but run slower and are less precise about exactly what broke. Using both together gives a team confidence at every level.

// Unit test, fast, checks one small piece in isolation
it('formats price correctly', () => {
  expect(formatPrice(9.5)).toBe('$9.50');
});

// End to end test, slower, checks the whole real user flow
cy.visit('/checkout');
cy.contains('$9.50').should('be.visible');