I'm having some difficulty on how to use the "stretch" optional argument of \parbox. Is it supposed to equally interspace lines of text inside a box of determined size?

If I code it like: 

```
\documentclass{article}
\begin{document}
  Text
  \fbox{\parbox[c][15cm][s]{5cm}{I want to stretch this text inside this box}}
\end{document}

```

This is the outcome I get:
![Example1](https://i.stack.imgur.com/ZyHbH.png)

I also experimented with \parbox and got some unexpected outcomes. In the following code, I merely deleted the alignment optional argument (default is center) from the previous test:

```
\documentclass{article}
\begin{document}
Text
\fbox{\parbox[15cm][s]{5cm}{I want to stretch this text inside this box}}
\end{document}

```

As a result, this came out:
![Example2](https://i.stack.imgur.com/x4pQA.png)

Which confused me, because I thought deleting the first optional argument would simply put it in default. I also didn't get why the text from the mandatory argument was substituted by the letter "s", a shortcut for "stretch" in the third optional argument.

Another test: 

```
\documentclass{article}
\begin{document}
Text
\fbox{\parbox[15cm]{5cm}{I want to stretch this text inside this box}}
\end{document}

```

Which yielded me:
![Example3](https://i.stack.imgur.com/8tSA4.png)

In this last one, I deleted the stretch optional argument, but the height of the box was not drawn.

Why did I get those results? I'm barely a starter at TeX, so might me missing obvious points on the use of optional arguments.