Pension contributions benefit from tax relief with the amount of relief depending on your income tax bracket. Every time you make a contribution, the government automatically adds a further 25% into your pension pot, and if you are a higher or additional rate earner, you can even claim back more. In effect, this means that money that would have gone to income tax goes to your pension instead.
You can benefit from this tax relief as long as you are within your annual allowance and provided you are not older than 75.
Moreover, any gains you make on your investments are not subject to tax as other non-pension investments would be.
See also our Tax Benefits of a Pension blog post.