1. at the start...to grab hold of any clothing thrown off just before the gun....
2. along the course..to cheer you on and throw you a drink....
3. at the end...to tell you how great you looked down the home straight?
4. everywhere!
When I did the Dublin Marathon a few years back SWMBO spent the entire race running across the city's back streets to meet me on the course so that I saw her about 5 times during the race.... she must have run a half marathon herself in providing this support.
I remember being really bewildered at the end of a race because there were so many people and i couldn't find my missus, which was quite depressing! But I still think along the course, it's really uplifting to have someone shouting for you in the midst of everything :-)
As for the question, only ever done one race and it was very nice to see people I recognised at regular intervals around the course, even if what they were shouting could only loosely be described as 'constructive'