I would assume that it is in the same rationale as "taking a minute of silence". It is a statement and a public ritual to both express and show seriousness of an event.
Obviously they are not the same thing, but it is not trivial to find a precise rationale under things that have no need to be 100% rational, such as showing public support.
Obviously they are not the same thing, but it is not trivial to find a precise rationale under things that have no need to be 100% rational, such as showing public support.