Why is variation beneficial to the species but not necessarily for the individual?

Explanation

Variation is useful for the survival even in adverse environmentla conditions. This happens as follows, There may be some drastic changes like excessive heat or cold or shortage of water (drought), etc., in the habitat of a species of organisms. Now, if all the organisms of a populations living in that habital are exactly identical, then there is danger that all of them may die and no one would survice under those conditions. This will eliminate the species from that habitat completely. However, if some variations are present in some individual organisms to tolerate excessive heat or cold or survive on meagre water supply, then there is a chance for them to survice and flourish even in adverse environment. For example, if there is a population of certain bacteria living in temperature water(which is neither very hot very cold) and the temperature of water increases too much due to global warming, then most of these bacteria will not be able to tolerate excessive heat and hence die. But some bacteria which had variations to resist heat would survive and grow further.