There could be multiple reasons why you don't want to download photos and videos in your mobile but want to view them.
- lack of space
- feel like spammed (like same images on festivals etc)
- don't want to take headache of deleting after seeing them
- restricted contents that you just want to see without storing in your mobile
There are two simple steps to achieve the freedom:
- Switch off media auto-download in your Whatsapp app by going to Settings -> Data Usage
- Use Whatsapp Web. Things opened in Web don't get downloaded in your mobile.
Note: This is how it is, at least, as of version 2.17.79.