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Because a file system arranges file in a tree structure (file system tree), when you want to read it, you perform a Tree - (Traversal|Search) which is therefore a recursion

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Example

Pseudo code

function scanDirectory(directoryPath){
   for (childrenPath in directoryPath) {
        if (childrenPath is a directory) {
            print("A directory was found"+childrenPath);
            scanDirectory(childrenPath); // the recursion
        } else {
           print("A file was found"+childrenPath);
        }
}

scanDirectory("."); // Scan the current directory