Insert>Picture into what?
Since you can use Insert Picture to insert a picture or a file reference to the picture file into a container field, I'm not clear one what you are trying to compare here...
I understand, perhaps incorrectly, that there are two ways to put a picture file into a layout:
1. create a new container field, choose Insert >file, find the picture file and open the picture file into the container
2. Place the cursor on the layout, choose Insert>picture, find the picture and open it.
Perhaps, I may have answered my own question: the former may just put the file into the layout, which must be opened somehow, probably by exporting it to the desktop, while the latter puts the actual picture in the layout.
Either approach can put a picture on your layout ready for view by the user. The result, in fact, is often identical from the user's point of view.
Where you have a difference is that if you want to place this picture on more than one layout, you can insert the picture into a container field and place this container field on multiple layouts. If you later need to change the picture, you can insert a new image once into the container field and all the layouts that use this container field will display this same updated image.
If you insert the image directly to the layout on several layouts, you'll have to go to each layout and insert the new image--which is more work and you now might miss a layout and then it doesn't show the new image.