Back before git, it was common to take a massive set of changes and merge them manually when merging branches, and BC was the only tool that made it painless for me.
I stumped up my own cash for a pro license and I use it almost daily even now.
It means you can do things like compare two system logs from different times and have it highlight you exactly the differences you care about.
Update: I just downloaded Meld. It literally takes ~half a second to change the cursor location when I click in a highlighted region, which is already making me dislike it. Is this normal?
My SO adores Meld, but she works with Linux. I tried using it but couldn't get used to it. She also makes fun of me (half in jest) because I use Tortoise Git instead of the command line...