There are two sides to this issue that you need to watch out for. On the one hand, don't reinvent the wheel. On the other hand, avoid licensing traps.
The internet is a great resource for getting code to do things quickly. There are all kinds of useful libraries and other utilities that make life easier. The issue of developer knowledge is certainly important and you do not want to be unable to withstand a code review because your code is replete with someone-else's. As Brian Kergihan once said: "Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it." Using code from people more clever than you are may well (and likely will) result in the inability to debug said code efficiently or effectively.
This leads me into my second point. Almost all code on the internet is licensed. Either explicitly or implicitly by copyright or similar. Certain licenses are very permissive as to what you can do with code, others much less so. The GNU GPL is famous for rendering any code it touches open source by default. Even one function could potentially render you obligated to release your codebase as open source software. Even Stackoverflow and the rest of Stack Exchange is licensed.
So by using other people's code willy nilly, you run the risk of both technical and legal shibolleths, the likes of which you may not understand.