(1) Employers are legally obligated to contribute towards your pension if you do, I believe the minimum is 5%/3% if I remember correctly. That means if you put in 5% (which in real cost will cost you 4% of less due to tax saving), they have to top it up with another 3% minimum at no cost to you. So essentially even a basic rate tax payer is immediately doubling their money through employer contributions.

(2) Beyond the employer matched amount mentioned above (and any additional incentives your specific employer may offer), any additional contributions will get 20% relief for basic rate tax payers, higher rate tax payers will save 40% / 45% respectively. (and actually potentially considerably more based on the hidden 60% bucket, NI savings and various schemes for parents)

(3) Even at 20%, by putting it into a pension you have that money earning for you throughout your working lifetime, rather than having that money in the governments hands.

(4) You get a personal allowance each year, so by drawing your earnings in the future via pension you benefit from that new tax free personal allowance each year you draw from your pension.

(5) There are benefits to having your money in a pension from an inheritance tax perspective, if required.

Basically pensions are excellent.