Why is it good to have a lot of variation within a population?

I'm assuming you are referring to a population such as a forest population. Appropriate answers would be 1) for a diverse food chain. 2) so the mechanics of an ecosystem can work and have multiple sources. IF you are referring to a population such as a given population of a species, it is important to have variety so mutations can occur and genetics can mix. This kicks off the survival of the fittest and Darwin's theory's. This, of course, takes plenty of time to see any change but genetic mutation is essentially survival of the fittest. If a mutation increases survival likelihood, they are now more fit to survive, thus creating a new "weak variation" in comparison to a new "strong variation" within a species. Read below for furhter explanation on previous stated points. I really hope this helps!

Explanation:

1) Populations of consumers would struggle if there is only one producer. If there are multiple variations of any producers, it will allow the consumers to thrive since there is an abundance and variation of producers. The same can apply to consumers since there are herbivore consumers and carnivore consumers. A population of wolves would struggle if their only food source were squirrels. If rabbits and deer were in the mix, wolves would thrive a lot easier.

2) If there was only one species of decomposer within an area, the decomposition process would take much more time due to a lack of efficiency with time. If there are multiple decomposers it wouldn't be much of an issue. Same goes with scavengers. There would be a lot of waste if only on scavenger lived within an area compared to multiple.