Layer > Bake Position keeps the current contents of a layer where they are, but normalizes the layer's scale and position so it is neither rotated nor scaled. It also fits the border of the layer around its contents, like Layer > Deflate To Fit Content.
This is particularly useful to reduce the resolution of a layer's image. If you import a very large image, say 4000 x 3000 pixels, into a 640 x 480 frame, Springboard will scale it down. It will look fine, but the extra pixels don't contribute to the appearance and may consume a lot of space in your file. Springboard saves the extra information because you want to enlarge the layer later to see more detail. But, if you don't need to enlarge the layer, you can Bake it to resample it at the frame resolution and save space.
You can also use Bake to increase resolution, if you've enlarged a very small image. It will smooth out the large pixels, which may improve the layer's appearance. And, it will give you more pixels to work with if you're drawing over it, either in Springboard or in an external editor.