Why do we use argument -i in rosdep install?

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What is the need to ignore the keys that are found to be catkin or ament packages anywhere in the path?

no, that's not what -i is for/does.

--ignore-packages-from-source tells rosdep to consider dependencies which it can find locally, in your workspace, as fulfilled.

In other words: if there are 4 dependencies, and 3 are already present in your workspace, rosdep will only install the single remaining dependency you don't have in your workspace.

Example: package.xml of pkg_c states dependency on pkg_a and pkg_b. Your workspace contains pkg_a. rosdep will only install pkg_b. pkg_a will be built as part of your workspace.

The usage text mentions "catkin or ament packages" as those are the only two types of packages rosdep would be able to detect in your workspace. You could have 20 other packages there (NPM, Conan, plain CMake, etc), but unless they are either Catkin or Ament packages, rosdep wouldn't know.