Nettle is a cryptographic library that is designed to fit easily in more or less any context: in crypto toolkits for object-oriented languages (C++, Python, Pike, etc.), in applications like LSH or GNUPG, or even in kernel space. In most contexts, you need more than the basic cryptographic algorithms; you also need some way to keep track of available algorithms and their properties and variants. You often have some algorithm selection process, often dictated by a protocol you want to implement. And as the requirements of applications differ in subtle and not so subtle ways, an API that fits one application well can be a pain to use in a different context, which is why there are so many different cryptographic libraries around. Nettle tries to avoid this problem by doing one thing, the low-level crypto stuff, and providing a simple but general interface to it. In particular, Nettle doesn't do algorithm selection. It doesn't do memory allocation. It doesn't do any I/O. The idea is that one can build several application- and context-specific interfaces on top of Nettle and share the code, testcases, benchmarks, documentation, etc.
