Beam angle dictates the area of illumination produced by a bulb.
For example, spot lights being used to illuminate small areas such as a shop display, would require a low beam angle, so as to strongly illuminate a small area. Whereas a higher beam angle would be required when used for illuminating a large space, the light emitted in this scenario would be less concentrated, and spread over the larger space.
For even light distribution when using down lighting within a space, the beams of the lighting fixtures should cross at roughly a 1.1m above the floor.
The higher the ceiling the narrower the beam angle necessary to illuminate the space, for lower ceilings a higher beam angle is recommended..