You can open your Word and Excel files in Pages and Numbers. The Apple applications are not clones of their Microsoft counterparts, and many features/functionality in the respective Microsoft applications/documents are simply not available in Pages and Numbers.
When you open the Microsoft Word and Excel files, they are translated into Page's and Number's internal document format — without modifying the original Microsoft documents. Any edits to the .pages or .numbers documents after the translation finishes are retained in the respective Apple documents, and not applied to the original Word or Excel document.
The exportation of Word or Excel documents from Pages or Numbers is again an approximate translation from Apple's internal document formats, and may not appear the same in the exported Microsoft documents due to features unique to Apple's applications, or limitations of the export process itself.
With Word and Excel documents, you are not restricted to one application's ability to open them. With Pages and Numbers, you are restricted not only to these Apple applications, but very likely the version of these applications that created them.