In theory, you should use unique_ptr for all pointers unless you know you want to share it, in which case you should use shared_ptr. The reason is that unique_ptr has less overhead since it doesn't count references.
However, a unique_ptr is movable but not copyable, so using one as a member variable can require you to write more code (eg a move constructor), passing one by value means you'll need to use std::move and so on. As a result some people use shared_ptr out of laziness, because it's just easier, and the perf difference may not be significant for their app.
Finally a raw pointer is fine for observation - pointer uses that can never affect lifetime. Careful choice of the right pointer type can give those who read your code a good understanding of what you are doing. For more, see Herb Sutter's essay, Elements of C++ Style, specifically the "no delete" section.
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