Byproducts are collateral outcomes from an activity which main purpose is not that product.
Many times, we produce things that we ignore or we do not take advantage of. The small parts that conform a big project can have value themselves.
“When you make something, you always make something else. You can’t just make one thing. Everything has a byproduct.” — 37 Signals
Work can be structured in order to force byproducts and plan what they will be used to. For instance, a book could be a byproduct of a series of blog posts, which initial intention was not a book. This approach provides benefits without having to do a lot of extra work, but being conscious from the start of potential byproducts can help making byproducts useful without extra effort.
An example is my blog’s Link Pack. It usually contains things I found on the web over that week, without specifically looking for content for those articles. I store them as they appear. Those articles are a byproduct because even if I do not use the links for the Link Pack I am still going to read them.
Byproducts can also help motivating you to accomplish big projects. Giving importance to small parts of a larger project —understanding those small fractions of the big project as byproducts— will help completing the whole thing with small steps, getting done the project in small chunks that are useful on their own.