Onboarding new developers into a project is a time-demanding and costly process, isn’t it? Just finding a proper specialist isn’t sufficient. We all know that every software development company has its own practices and uses its own tools. And those tools might even be in-house developed.
Therefore, it doesn't matter how skilled and experienced your new team member is, they will need to learn a lot. Sometimes, it takes pretty much time for them to learn all about the project they start working on (to start coding new features, for example), about the company’s corporate culture, and the commonly accepted practices and processes. Needless to mention the extensive documentation, and similar.