I thought this is quite simple?

You should start a race in your estimated finishing place. Same applies to a fell race/5K/10K or whatever. Everyone needs to get into their own running pace asap - not too fast nor too slow..

This is important, e.g. for me this year trying to run sub 40K and wanting the first Kilometre to be 3.50 - 4.00, you don't want the first 300/400m getting past runners with ipods on carrying litres of water with running jackets on.
Yes, it can be dangerous too, with pushing and tripping, which can be easily avoided.