The advantage is you get bundle discounts, mystery boxes, flash sales (which isthereanydeal may miss as they poll only once a day) or other unusual sales which don't show directly as a discount on the games pages. Also isthereanydeal sometimes screws up with special editions and the prices get merged when the titles are too long.

For the traders, t also may give you an idea what they paid for a game. It might be a $39.99 game, but if it was bundled for 90% off, do you really want to trade something that has only been 50% off?

Worth a bump

Needs more bumps and +'s!

I think I've spotted an error in the data, there's the following entry for Time Commando:

Time Commando $5.99 75% $1.49 6/21/2016 Summer Sale 2016

but it's currently on sale for 1.19$ / -80%

Thank you! Will be corrected in tomorrow's update. (I use a working copy and transfer it when done. Have the new DLC releases added too, but they can wait for the morning.)
It's a fine site, but it also has it's drawbacks. It will not capture Flash Sales, Time Machine Sales, or Insomnias unless it gets lucky and checks during that particular sale. It does not capture bundle deals at all, only single game prices, and this is a specialty of GoG. And it will completely fail to catch things like Mystery Boxes.

It's a useful tool, but I've tried to make this spreadsheet cover the complete gamut of deals in as concise and easy to search form.