Question: What are the reasons or security reasons why a URL would be linked this way 

```
http://www.sampledemo.com/redirect?id=17&link=http://www.sampledemo.com/manual.php

```

instead of this way

```
http://www.sampledemo.com/manual.php

```

Thank you

Answer: 
Probably it isn't being used for security purposes. It might be used for site metrics and click tracking. It's hard to say what the `id` parameter is for. Could be something like an ID of the page being linked from.
There are several reasons, but the most common one is that you want to track where you're being linked *from*. See the `id` argument? That very probably corresponds to the site.