As most people know - the Boston marathon is the oldest annual marathon in the world. It was first run in 1897 - one year after the innagural Olympics. Given the popularity of marathons these days, you might be suprised to know that of the four other "major" marathons (New York, Berlin, London, Chicago) the earliest is New York, which started in 1970. Before the running boom of the 1970's marathoning was not very popular - Boston survived based on tradition.
To get more of an idea of how a non-runner Bostonian views the marathon - this "idiot's guide to the Boston Marathon" by ESPN's the Sports Guy is pretty good. It's not my favorite column of his - but, there are some good lines like - "Any athletic activity that causes you to pee on yourself, justify it and have the justification actually make sense is something I don't want to be doing under any circumstances."
Basically he says that the Boston Marathon is an event (~500,000 people spectate) because people get the day off from work (Patriots Day), it gives people an excuse to be outside when the weather is finally getting good, get drunk, and make fun of other people. Four things that Bostonians love to do apparently.