botany - Why do plants produce so much more sugar than they use?

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?