Why and when to inherit from Collection<T>

I'm tracing legacy code in my project written in C#.

I find the following code:

public class FooCollection : Collection<IFoo> {};

I don't understand why (and when) we need to create our own Collection class like this.

Why not just use the build-in collection (array, List<T>, Dictionary<K,V>, ...)

What's the data structure used in Collection<T> ?? array? or linked list?