Well, almost any big traditional application (not really web apps) eventually grows a scripting language for extensions and / or user macros. Basic (in Office), Javascript and Java (in web browsers), Lisp (in the above applications, and probably many more), Lua (in a lot of games), AppleScript, that C thingy in Quake ...
Lisp is relatively easy to implement, powerful, and intuitive to non-programmers (compared to OOP - Python, Perl, and Ruby are also easy but nowhere near as old); so it's a common choice.
I didn't believe this "Easy to implement" line but over Christmas I wrote my own LISP implementation on top of .NET and it comes to about 350 lines codes.
That includes all the list operations, an arbitrary precision number type and a host of the usual built in function (add, subtract, multiply, divide, and not or etc).
Lisp is relatively easy to implement, powerful, and intuitive to non-programmers (compared to OOP - Python, Perl, and Ruby are also easy but nowhere near as old); so it's a common choice.