If someone makes high enough pension contributions, or a has a very high income, they may need to worry about the annual allowance.
They need to add up all both employee and employer pension contributions for an entire tax year to check if they've exceeded the allowance.
Many people have employers that directly make pension contributions for them, and also operate a salary sacrifice scheme to allow the employee to make contributions that save both tax and national insurance.
If contributions are recorded in one tax year but received by the pension scheme in the next tax year (e.g. they show on a March payslip but received after April 6th), which tax year do they apply to for annual allowance purposes?