Anthocyanin belongs to flavonoids. As far as I know, it can be found in many bryophytes that have evolved for 470 million years, so it should be an ancient pigment that has withstood the test of evolution. First of all, anthocyanin belongs to a pyran salt. This large yoke system will definitely develop color. The positive charge of this system can turn phenol into quinone (similar to phenolphthalein) under alkaline conditions, and in acidic conditions, water can be added to block part of the yoke system, thus almost all colors can be obtained. The main use of anthocyanin color is warning color. It is said that many insects don't eat those red leaves. Moreover, anthocyanins are related to defense against threats. For example, a tobacco pest named Heliothis virescens has reduced its fertility after eating infected leaves that turn red. Second, anthocyanins are polyphenolic compounds. This may be the original antioxidant. As an antioxidant, anthocyanins can deal with too much reactive oxygen species (oxygen may destroy lipid compounds) under too strong light. In addition, polyhydroxy compounds can chelate metal ions to a certain extent, and also have resistance to heavy metals and other environments. Thirdly, anthocyanin is a strong ultraviolet absorber. It can also do something about UV, a dangerous thing that can cause [2+2] reaction of bases. In addition, experiments show that it is effective in resisting overheating and supercooling environment, promoting electron transfer in photosynthesis and resisting flooding. In short, anthocyanin is a kind of thing that has too many functions. It makes sense for plants to choose anthocyanins to exist in their leaves. By the way, this thing finally survived, of course, because anthocyanins helped plants adapt to the earth's environment. Those lucky plants who made it first passed it on. The above functions are definitely not "purposefully" directed.