The main upside is that it's just regular code. This is more natural for some folks, but mainly the advantage is that you can build abstractions. So, you can build a component that's built up from lower-level resources in AWS. For example, directly setting up API Gateway requires a specific setup (and can be error-prone), so there's a Pulumi library for that.
My colleague wrote a blog post with more of the technical motivation: https://medium.com/@lukeh/programming-the-cloud-e795cafffc2b, and there's also this Twitch live-coding video: https://youtu.be/DM8Wd4f1MNA