It's helpful for applique, especially tracing. I've also used it to help line up fabric for paper piecing (though now that I'm better at that, and using a different technique, I don't need it like I used to).
My sister-in-law is a veterinarian, and she loaned me a portable x-ray film light (it's about the size of a clipboard and just a little thicker). It's awesome! They don't use them anymore (it's all digital now), so she took them home. I'm not sure if I have to give it back or not...
I did find some inexpensive ones on amazon though, around $20 give or take. I know there is a Crayola brand one (for kids and tracing) as well as some other brands, and they work well too.