In any given software developer's career, there are many different things they need to create. Of the software they need to create, given time constraints and resources, it's almost impossible for them to perform the research involved to produce everything correctly from scratch.
There are all sorts of barriers and roadblocks to researching how to correctly write this bit of code or that bit of code, use that technology, or this interface. Documentation may be lacking or missing, or documentation may be completely wrong. Documentation is the same as software, sometimes it has bugs. Sometimes the act of writing software is unit testing documentation. This, of course, provides no value to most software development projects. It's great when the documentation is correct, but when it's not, it can be devastating to a software project.
Even with correct documentation, sometimes we don't have the time to read all of the documentation and become total experts in some technology or API. We just need a subset of the API to do what we need done and that's all.