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.
If the URI was properly encoded (it isn't), then the only reasons I can think of are logging the hit and/or displaying a message to the user before they click to continue.