Yes, when saving to JPEG, the digital zoom is useful! Looking at the answers, it is surprising how many people is not aware of it!
you will achieve better quality. Just make a test and you will see. There is a JPEG artifact, which works at few-pixel scale, and this artifact will be enlarged as you will crop and zoom the image in post-process. It is especially strong for lower quality settings. When you zoom in the camera, the pixels are enlarged so they are not affected by the JPEG artifact.
you will save space on the card. If you do digital zoom instead of keeping the full frame, then
a) you don't need the largest JPEG quality setting to have a good quality crop later
b) the resultant JPEG got from optical zoom will be much smaller then the full frame, even if it has equal dimensions and quality setting - because the picture is more blurred, more uniform, so it can be better compressed with JPEG.
1) you will achieve better quality. Just make a test and you will see. There is a JPEG artifact, which works at few-pixel scale, and this artifact will be enlarged as you will crop and zoom the image in post-process. It is especially strong for lower quality settings. When you digital zoom in the camera, the pixels are enlarged before they are first saved to JPEG so they are not affected by the JPEG artifact.
2) you will save space on the card. If you do digital zoom instead of keeping the full frame, then
a) you don't need the largest JPEG quality setting to obtain a good quality crop later
b) the resultant JPEG got from optical zoom will be much smaller then the full frame, even if it has equal dimensions and quality setting - because the picture is more blurred, more uniform, so it can be better compressed with JPEG.