You might as well say, why does anyone ever design a new chair - or come to that, any piece of furniture - at all? Haven't we worked out what's good to sit on yet? Are there not timeless post-war classics from Arne Jacobsen, Charles and Ray Eames, Robin Day, Rodney Kinsman and shoals of others?

True enough. You will instantly recognise their chairs when you see them, even if some of the names behind them are maybe a little unfamiliar at first. Their products can still be bought today, as can the chairs of the new wave of British designers who emerged during the 1980s, among them Jasper Morrison, Ron Arad, and - begetter of the new Wait chair - Matthew Hilton. And, of course, there is always the prolific French designer Philippe Starck, whose creations (with such memorable names as Dr Glob and Lord Yo) acquire cult status the moment they are launched.

But chair design can no more stop evolving than architecture can. Somebody, somewhere, was one day going to get to grips with the mass-produced plastic chair and make something better - but still affordable. Many have tried. But Matthew Hilton has succeeded. It is no small achievement. It is a William Morris moment: good design for the masses.

Wait is launched at this week's Milan Design Fair - the biggest, glitziest annual international event of its kind. As at motor and fashion shows, there will be plenty of flashy stuff by other names that will grab most of the press attention. Wait is different. It is the equivalent of an advanced new hatchback or a sportswear collection. It is not "concept", it is not couture. Its cleverness lies not in its appearance - which studiously avoids any whiff of trendiness, unlike Starck's costly transparent attempt at an all-plastic chair - but in its structure. It is strong, it is light, it is stackable, it is a single injection-moulded piece of polypropylene. The technology and the material is the same as for all those bendy garden chairs. The only difference is design.

It goes on sale in Britain in May. It is made by a German plastics company, Authentics. It will cost about £30. Again, this may not seem earth-shattering - you can get crude plastic chairs at petrol stations for half that - but the fact is that "designer" chairs, even plastic ones, tend to cost at least £80, and more usually well over £100. Hilton's previous designs, many of them acknowledged modern classics, such as his sculptural Balzac leather armchair and sofas for the British contemporary furniture company SCP, gave little hint that he was seeking the Holy Grail of mass production. But then Hilton went on holiday, and found himself sitting on those ubiquitous cheap and nasty plastic chairs. "I can do better than this," he thought. And he spent the next two years doing it.