The 20% (in actuality, I think it is 18%) bonus is certainly not ungenerous, but it is also only about £2,600 per person on average. Average salaries for the 70,000 staff are only £13,000 p.a., which suggests there are plenty of staff on much less than that.
If I was earning less than £13k and my employer gave me a couple of grand extra, it would make a very substantial difference to my quality of life. As a management consultant with a healthy salary, I'm not sure I would see the effects of a 20% bonus as directly in my quality of life (for a start, much more of it would go to the taxman).
However, it does show what kind of difference it makes when everyone is considered to be a partner in the firm. At John Lewis, the 20% bonus applies to everyone and there is proportionally less disparity between the top and bottom of the organisation than you'd find at most professional service firms.