Wikipedia states that erythrocyte aggregation causes disease:
Conversely, the presence of rouleaux is a cause of disease because it will restrict the flow of blood throughout the body because capillaries can only accept free-flowing singular and independent red blood cells.
but also it states that the body has special proteins that cause it:
The most important protein causing rouleaux formation in plasma is fibrinogen.
Does erythrocyte aggregation serve a useful function?