Waves are typically configured for triathlons, because they usually do not have a timeline at the start of the race. Therefore you will need to start the race with a gunshot. You can configure multiple waves and start each separate wave with a gunshot. Alle athletes assigned to this wave will get the gunshot as their start time.
Marathons and other races can be tracked by chiptime. Only when you want to show guntime for each wave, or for example give you have to create waves. The reason you might not want to create waves for such a race, is that athletes do not necessarily start in the correct wave and might then get a wrong start time, for example because they started in an earlier wave than we expected and their chip time start is then seen as a false start.
For elite athletes you might want to set up a separate wave and give all athletes in this wave the gunshot as start time.