Let's Play videos are great for the gaming industry, specially indie game developers. For example, Minecrafts success was partially due to all the Lets Play videos that were created on Youtube. Its essentially free publicity for video games. The prime example of this would be Seananners[1] who made a video that currently has 1.5M views. In addition, Lets Play videos allow players to preview the game before they buy it to get to know the game and see if its worth purchasing. A trailer is not always enough.
Disclaimer: I launched a website dedicated to Lets Plays 2 days ago (http://commentato.rs)