Scripting Languages on top of programming languages

So I've had this question lingering around in my head for quite a bit and it's starting to bug me. Many projects (mostly corporate) use many languages to work on one project or codebase. I understand the need of this in web development projects (php, js, etc), but not really for others. Why would I need (or want to use) a scripting language such as Python or Lua and not just write my whole application in something like C or Java?
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