This list may include games that have passed Greenlight but haven't yet been released on Steam.
Some reasons you may want to purchase a game on itch.io instead of directly from Steam:
In most cases you also receive a Steam key in addition to a DRM-free copy from itch.io.
The developer receives more of the profits. Steam takes 30 percent, but developers can choose how much of the revenue they want to share with itch.io from 0 to 100 percent (default 10%).
Price. If the store is taking a smaller cut of the revenue, the developer may be motivated to offer the game at a lower price. Don't forget to compare prices and watch for sales!
You're encouraging places like itch.io to exist, where the creators are in control and can spend time making stuff instead of wasting it campaigning for votes just for the privilege of selling their work.