It is tricky. In terms of finding specific ways to differentiate a firm,
@Alice G gave some great suggestions for areas to look out for:
https://www.thecorporatelawacademy.com/forum/threads/struggling-to-differentiate-between-firms.1332/. You want your reasons to show that you understand the kind of firm it is and be specific enough that it doesn't just apply to every commercial law firm out there (e.g. buzzwords/cliche phrases like 'cutting-edge deals', 'impressive clients' and 'intellectually-stimulating work'). That said, it doesn't need to be anything groundbreaking and it's fine if there is some overlap with other firms; it should just be persuasive and clear.
When linking it to yourself, it's great if you can be personal, although not all points will lend themselves to that. You could just be very specific as to why a particular reason makes you want to apply.