Some sites automatically play Flash videos which can be annoying, troublesome at work and suck up bandwidth. The Chrome extension FlashControl stops Adobe Flash from loading unless you have authorised a particular site to bypass the restriction.
The extension also turns off Flash when the computer is inactive for a length of time or when a tab with a flash video is inactive. There is a wide range of controls for these options so you should be able to easily dial these settings in to your browsing preferences.
The extension works on the principle of whitelisted and blacklisted sites. All sites are assumed blacklisted until you change the settings.
If Flash videos automatically playing is a pet peeve you should go ahead and install the extension.