It isn't uncommon for a company to put on a sale where the product is being sold at a loss. For Valve, this seriously benefits them as Steam is likely undercutting all the competition at the time, bringing in more customers for full-price products. In the case of both Valve and the company that developed/published the game, this also has the benefit of more people buying the game at full-price after the sale as those pleased with their purchase during the sale will start spreading the word about how great the product is. The thing is, while there are plenty of customers that wait for good sales, there are plenty that are willing to buy while the game isn't on sale, making up for those who buy while one sale.
Also, while Steam is well-known for sales, it should be said that they aren't the only ones who run sales like they do. You can almost always find a few products at stores on sale, and anyone who's ever shopped for electronics knows summer is a great time to do it because of all the "Back to School" sales. The difference between Steam and a company like Best Buy, Amazon, or Walmart is that while Best Buy, Amazon, and Walmart have a vast array of products they can put on sale, Steam has a very focused product list (basically, nothing but games). It also helps that Steam heavily advertises all its sales products, while going to a retail store requires you to look around for the products on sale. Even on their websites, you generally have to go to the product category you are looking for before you can really start seeing the sales, unless, of course, you are signed up for the company's newsletter. This isn't Steam putting on more sales because they are more financially capable of doing it due to the presumed lower costs of digital distribution. It is simply them following a common business practice on a very focused product list.
As for myself, I generally buy on sale, but if it is a game I'm really looking forward to, I may buy day one. If it just had a sale I missed but people are really getting excited about it, I may also buy it at full price.