I recently asked the question, "Do plants need O2 to consume energy they've stored via sugar?" to which @canadianer responded, "Yes, plants require oxygen to generate ATP from sugar. However, they generate far more sugar (and O2) than they use." Why do plants produce all that extra sugar, especially given that sugar is thermodynamically expensive?
One reason, I believe, is to produce fruit and so recruit animals to help with reproduction. But are there others?