It's called "marketing" and a few developers do a great job of it -- others obviously do not. Since I look at mostly media apps I have to say that the media companies do a particularly bad job.
I don't know if a "sample" is what is called for, but certainly a YouTube video that demonstrates the app in action would be nice.
I think Apple should allow developers some sort of option where you an try a full blown app for 30 minutes and then buy it within the app. I don't blame developers who don't throw out a Lite version since that takes extra development time to throw together.