Adding to Dale Emery's answer, besides the effects listed in Wikipedia, one basic effect is that of emphasizing the staggering number of things:
- "there were apples, oranges, and bananas" is just a neutral list,
- whereas "there were apples and oranges and bananas" emphasizes the fact that there are impressively many different fruits to choose from.
There are many common phrases that emphasize endlessness through the repetition of conjunctions:
It went on and on and on forever.
and is just the standard end of a list, several
ands are repetitive, and repetition conveys monotony and sheer volume.
So, to use your own exampes,
"shrubs, trees, and bamboo shoots"
means that there were these three types of plants (and no more or nothing else of significance), and you're stating that in a neutral manner, while
"shrubs and trees and bamboo shoots"
means that there was an impenetrable jungle of plants that contains the three plants and many others, and your narrator is very emotional about that fact.