Mockups with PowerPoint are just not effective. Also, with apps that are more than just a couple pages, it quickly becomes cumbersome, even inside Photoshop or Illustrator.

With Balsamiq, you're able to think out loud and get more decisions made faster. Also, when you're working in a team and not 100% clear on what the solution you're building will end up becoming, it helps a lot with the communication.

Lastly, when you know what you're building, it takes a lot less time building it (at least for me it does.)

For example, we're about to build and launch a new product in the next couple weeks. By spending some time creating a wireframe prototype that the whole team can click around in (https://volcanic.mybalsamiq.com/projects/customerloyalty/gri...) we've been able to tweak what we want version 1 to be and narrow our scope quiet a bit to just the essentials.

I'd say wireframing, particularly with myBalsamiq, is very beneficial for web app development.