They allow multiple particle streams to transform into new particles. No particle system would be complete without some kind of external forces to influence particle motion. Global fields are found in the System panel and they include directional gravity, wind, electricity and magnetism. Electricity works like the Push space warp by applying positive or negative force to particles along a vector. Magnetism is a vortex-like effect causing particles to swirl around.
Gravity is the only global field that affects particles absolutely.
The others influence can be controlled by parameters within the particle rollout. Wind only affects particles with positive or negative drag. Along with the global forces, there are localized versions of Electric and Magnetic. These only affect particles relative to their areas of influence. I ve done some complex swirling particle motions with groups of localized magnetic fields.