Why do people replace diesel engines in a sailboat?

[Chotu]

Another thread got me to thinking. Why is it that people decide to re-power?

Assuming you have a working engine, a diesel engine, what is the point?

I see a lot of people pull out older engines and put in a brand new one. I’m thinking of ones like an old Perkins. Sure, they look old. And they probably don’t get as great fuel efficiency as the new ones. But why replace them if they are still running?