When I should use find_package - cmake

Question

I am learning CMake, and I feel hard to understand when I should use find_package.
For separate compilation, we need to let the compiler knows where to find the header file, and this could be done by target_include_directories. For linking, we need to let the linker knows where the implementation is, and this could be done by target_link_libraries. It seems like that is all we need to do to compile a project. Could anyone explain why and when we should use find_package?

Answer 1

If a package you intend allows for the use of find_package, you should use it. If a package comes with a working configuration script, it'll encourage you to use the library the way it's intended to be used likely come with a simple way to add include directories and dependencies required.
When is it possible to use find_package?
There needs to be either a configuration script (<PackageName>Config.cmake or packagename-config.cmake) that gets installed with the package or find script (Find<PackageName>.cmake). The latter one in some cases even comes with the cmake installation instead of the package installed, see CMake find modules.
Should you create missing scripts yourself?
There are several benefits in creating a package configuration script yourself, even if a package doesn't come with a existing configuration or find script:
The scripts separate the information about libraries from the logic used to create your own target. The use of the 2 commands find_package and target_link_libraries is concise and any logic you may need to collect and apply information like dependencies, include directories, minimal versions of the C++ standard to use, ect. would probably take up much more space in your CMakeLists.txt files thus making it harder to understand.
If makes library used easy to replace. Basically all it takes to go with a different version of the same package would be to modify CMAKE_PREFIX_PATH, CMAKE_MODULE_PATH or package-specific <PackageName>_ROOT variables. If you ever want to try out different versions of the same library, this is incredibly useful.
The logic is reuseable. If you need to use the same functionality in a different project, it takes little effort to reuse the same logic. Even if a library is only used within a single project, but in multiple places, the use of find_package can help keeping the logic for "importing" a lib close to its use (see also the first bullet point).
There can be multiple versions of the same library with automatic selection of applicable ones. Note that this requires the use of a version file, but this file allows you to specify, if a version of the package is suitable for the current project. This allows for the checking the target architecture, ect. This is helpful when cross compiling or when providing both 32 and 64 bit versions of a library on Windows: If a version file indicates a mismatch the search for a suitable version simply continues with different paths instead of failing fatally when considering the first mismatch.

Answer 2

You will probably find CMake's guide on using dependencies helpful. It describes find_package and alternatives, and when each one is relevant / useful. Here's an excerpt from the section on find_package (italics added):
A package needed by the project may already be built and available at some location on the user's system. That package might have also been built by CMake, or it could have used a different build system entirely. It might even just be a collection of files that didn't need to be built at all. CMake provides the find_package() command for these scenarios. It searches well-known locations, along with additional hints and paths provided by the project or user. It also supports package components and packages being optional. Result variables are provided to allow the project to customize its own behavior according to whether the package or specific components were found.
find_package requires that the package provide CMake support in the form of specific files that describe the package's contents to CMake. Some library authors provide this support (the most desirable scenario for you, the package consumer), some don't but are prominent enough that CMake itself comes with such files for those packages, or in the worst case, there is no CMake support at all, in which case you can either do something to get the either of the previous good outcomes, or perform some kludges to get the job done (ie. define the targets yourself in your project's CMake config).