It is true that it's a buyers market and many Unis will take grads below the offer in August. However, unless you're going through Clearing you need the offer in the first place and that's why predicted grades are so important.
A levels are effectively a currency. Lesser Unis cannot demand so much and have to be more flexible on offers day. Yes, one that made offers of CCC won't expect to fill many places with CCC but to be taking a range of Ds and Es. Those offering BBB will expect many candidates to have at least 1 C in the mix of their results.
When it comes to results day, Unis need to fill up to get the money in. They would rather accept someone they have already offered to and who made the uni their 'firm' choice on their UCAS form and so is likely to accept and turn up, than take the risk if turning them down in hope of someone else who will have higher grades, but might not materialise. Filling places fast is the name of the game, because the more days that pass, the less good candidates remain.
So actually, I think the grades suggested in prospectuses are a bit misleading in terms of what people on the courses actually have. In the majority of cases it's a bit lower. For Oxbridge and one or two other places and very popular courses it's the opposite and people actually have far in excess of the 3 As mentioned or made as the standard offer. For Oxbridge, students at the application stage sometimes see the offer listed in the prospectus as AAA and if they are scraping close to that kind of prediction think they stand a chance...but don't realise the profile of those who actually go is much higher than that.
Not everyone can be a high flier. Middling achievers can get to RG Unis if they pick less popular courses. And there are lots of Unis which take much lower grades and most people who want to go seem to find a place. Whether it's worth it for the level of debt and job opportunities from some of those courses and whether all who go are really suited to degree level education or degree level education at 18 is a different question. Lots of the lower achievers, especially when they have been immature and not worked hard or faced particular issues, would benefit from waiting a couple of years or longer and going when they are actually ready to study hard and will get the most out of it.