why do plants have more than one kind of chlorophyll?

Answer 1

The original phototrophic organisms where aquatic. Light absorption under water is best in the long penetrating red wavelengths where alpha mostly absorbs. The other pigments arose later to capture the high energy blue light first in shallow water and on then on land.
Chlorophyll alpha does the primary electron capture to process into carbohydrates for long term storage of energy. Alpha absorbs reasonably well at a wavelength of about 450 nm but absorbs most at red 675nm.
Accessory pigments can include chlorophyll b, xanthophylls, and carotenoids (such as beta-carotene). They absorb energy that chlorophyll a does not absorb and pass it to alpha.

Chlorophyll beta absorbs most effectively at blue 470 but also at 430 and 640. Beta differs from alpha only by its 7-formyl group. Its main function is to collect energy but a secondary function is to regulate the antenna size. Alpha occupies the reaction center of the antenna array. The array is made up of the core proteins surrounded by the peripheral proteins. Core proteins bind chlorophyll a and carotenoids. The peripheral proteins in land plants bind both alpha & beta.
Chlorophyll beta is found only in peripheral light-harvesting complexes (LHCs). These complexes usually contain three xanthophyll molecules, two luteins and one neoxanthin, and nearly equal amounts of Chl alpha and Chl beta.
Chlrophyll beta-less mutant plants contain fully functional reaction centers but have a relatively low photosynthetic capacity and greater sensitivity to high-intensity light.

Carotenoids reflect yellow, orange, or red and absorb blue light energy between 400-500 nm.
Xanthophyll absorbs well at 400-530 nm

Answer 2

Each of the different kinds (from what I remember, there are two chlorophylls, and two other pigments) absorbs a different part of the light spectrum.

So, say (not sure if this is right), Chlorophyll A absorbs red light, and converts it to chemical energy for the plant, and Chlorophyll B does the same with blue light. This is all invisible to the naked eye, of course, but the red and blue light comes from separating the white light we get from the sun.