Firstly, because he won Gold medals at both the 10,000m *and* the 5,000m finals in the 1972 Summer Olympics in Munich (known as 'the double'). Only 6 people have ever achieved this (including Ethiopia's Bekele in Beijing a year ago).
Secondly, Viren was remarkable because he broke the world record in that 10,000m final, which had stood unchallenged for seven years. And he did this despite stumbling and falling flat onto the asphalt on lap 12, instantly dropping nearly 30m behind the leaders. He got back up, and swiftly narrowed the gap with astonishing ease before going on to outclass the whole field - and snatched the world record into the bargain.
I'm an amateur 10k runner, so looking at his time of 27:38 minutes for the 10k seems impossible - I'd need almost half his time again to complete that distance (and that's without falling over).
Thirdly, unbelievably, he did it all again four years later at the 1976 Montreal Olympics - Gold in both the 10,000m and 5,000m. By doing this he had achieved the 'double-double', a term they had to create specifically for his achievement. No one else has ever achieved that, before or since.
His speedy finish in the 5,000m meant that his last 1,500m would have given him a respectable placing in the 1,500m final itself. The day afterwards, for the hell of it, he tried his luck at the Marathon and finished it in a snippy 2 hrs 13 minutes.
His secret to doing so well was no accident, and this is where the lessons for me were striking.
Firstly, he trained in local forests. Not because of a fetish for shrubbery, but because, as he put it, "you have to change rhythm to avoid roots, just in the same way as you have to be constantly alert in a competition". So he didn't just run around a track to get better at running around a track. He deliberately trained in difficult circumstances and outside of his comfort zone - and neither did he over-rehearse in the arena he would letter compete in. So when something surprising happened on the track, he could deal with it (such as tripping up).
Secondly, he prepared with absolutely one thing on his mind - the Olympic Games. He had a four-year-long training cycle, increasing intensities over the intervening years so that he would peak at the Games. Everything else, every other championship was just practice - all building up to one special moment. One goal, one date, with everything primed towards performance at those two distance finals.
In later years he became an MP in the Finnish Parliament. I don't know much about Finnish politics and what he achieved in his political career, but something tells me it was full of exciting ups and downs, but he probably had one major policy focus in his mind.