Developers (of malware as well as goodware) don't have to reinvent the wheel all the time. Why rewrite a piece of code that was development by someone else? In the same way, all operating systems provide API calls (or system calls) to interact with the hardware (open a file, display a pixel, send a packet over the wire, etc). These system calls are grouped in libraries (example: Windows provided wininet.dll to interact with networks).
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