The main reason to use a PRSA is as an alternative to other pension provision. Usually, a company provides a pension package as part of the job offer but not all do. This is especially common with smaller companies or new startups.
Sometimes people choose to take on a PRSA in addition to their company pension provision. This is legally acceptable but you do need to consider whether you need it or whether it might be more beneficial to make voluntary contributions to your company pension.
Not all company pensions allow voluntary contributions so this would be one scenario when it might make sense to have a PRSA as well as a company pension.
If you are on a Defined Benefit Scheme you will be guaranteed a specific payment when you reach retirement age. Many people on defined benefit schemes do not feel they need additional insurance as their future income is assured and based on their current income. However if you feel that it might not meet your living costs in the future then do consider whether a PRSA might help you close the gap between future costs and income.
If you are on a Defined Contribution pension scheme you may also not need a PRSA as you are already taking the investment risk with your existing plan. Take advice from an expert pension advisor before transferring to a PRSA as this may affect the matching payments made by your employer.