Paper documents tell you what they say. Electronic documents tell you what they say and who created them, when, on which device, who else received them, whether they were edited, and how they got from one custodian to another.
That metadata is what makes ESI powerful in litigation: authenticity, authorship, and timing become objective facts rather than disputed testimony. Combined with full-text search across millions of pages, electronic review can be orders of magnitude faster — and more thorough — than traditional document review.