Payments on account have existed for as long as Self Assessment has existed. There are two criteria, both of which must be met, for payments on account to be created. The first is the underpayment of tax is more than £1000.00. The second is that 80% or more of the underpayment cannot be collected through an automatic source.
Payments on account for the next tax year are based on the underpayment of tax for the previous tax year, as a starting point. Circumstances may change the fortunes of the business etc and the payments on account need to go up or down. Ideally, both payments on account will cover the tax liability for the whole tax year, when the tax return is submitted and the tax due is calculated.
There is some guidance on payments on account and SALF300 here
Understand your Self Assessment tax bill
SALF303 - Payment of tax: payments on account