Primers are designed to perform three main tasks; seal the substrate to reduce penetration and air release from concrete pores and pin-holes, consolidate and strengthen an otherwise weak or dusty substrate and to improve adhesion of the flooring system.
A floor seal can often act as its own primer, two coats usually providing the full system. Flow applied flooring for example benefit from the use of a primer (double priming in some cases) by sealing small air pockets in the concrete base due to pores and pin-holes and therefore reducing the risk of these bubbles rising to the surface and spoiling the smooth, impervious finish which is the ultimate goal.
Resin screeds tend to be dry mixes which are almost impossible to apply and compact satisfactorily unless the material is applied into a tacky primer or one that has been 'seeded' with a material like dry silica sand. Even the heavy duty flooring will normally benefit from a primer by reducing pin-holes and increasing adhesion.
Primers also serve to seal contaminated substrates, again increasing adhesion and prevent contaminants damaging and de-bonding the whole system. Special primers are available to deal with problems such as oil contamination and damp substrates.