أَبْصَرْتُهُ بِعَيْنِي
I saw it with my own eyes.
كَتَبْتُهُ بِيَدِي
I wrote it with my own hand.
Seeing ordinarily involves the eyes and writing usually involves the hand. Yet the extra phrase may distinguish direct observation from hearsay, or personally writing from having something written by another. In those contexts it performs emphasis or specification.
Where no such contrast is relevant, its benefit is less apparent. The lesson is that useful expansion and padding cannot be separated without considering context.