The short answer is it hepls with the car over steering if you loose control.
Although if you have a Mk2 MGF VVC it is recommended to have all four tyres the same size and profile.
Yes. That's the official reason. Supposedly because most people were used to front wheel drive, it was MGR's attempt to reduce the effect of rear wheel drive.
In my opinion, it was probably more to do with saving money, and all it actually did was confuse people. So you have people fitting the wrong sized tyres or having mismatched tyres on axle sets.
My mk2 F freestyle has 215/40 16 all around as standard, and I've never had an issue with oversteer.
The first car to have the thinner fronts was the Trophy.