Warming filters reduce excess blue tones that effect daylight film when shooting sky or sea scenes or in high altitude photography. They are also effective if your flash exposures are too 'cool'. 'A' is the mildest of the 81 series. It has a filter factor of 1.4 and requiring 1/2 f-stop of additional light.
All light sources, natural or artificial, have characteristics that affect color rendition on film. Color correcting filters are designed to 'warm', 'cool' and/or correct for the many variations of lighting you might encounter.
All filters have a 'filter factor' which is a measurement of the light it absorbs. Cameras with internal metering (through the lens) will automatically adjust the exposure. With external or hand held meters you must allow for this factor.
Note: Modern strobes (flash) approximate midday sun and any warming or cooling filtration with flash should be based on 'daylight'.