Pinnipeds need to molt from time to time because their fur doesn’t keep them warm but just protects them from injury and getting burnt by the sun’s ultraviolet light. Also, they need to molt mostly on land because they need to send their blood close to the surface of their skin to encourage the growth of new fur. They couldn’t do this in the cold waters where most seals live because the blood would be shunted to their internal organs. With their fur and even their skin sloughing away they would become hypothermic very quickly. A seal that still visits the water during its molt takes a much longer time to get a new coat of fur.
Many pinnipeds molt right after they become pregnant, even though elevated pregnancy hormones can interfere with the molt. The animal gets around this through a phenomenon called embryonic diapause. While she’s molting, the embryo simply doesn’t develop and the hormones are kept in check.
There are animals that don’t just lose and regrow their fur but lose and regrow their top layer of skin. Elephant seals do this, and they not only have to stay on land for some weeks, but they can’t eat during this time. This is called a catastrophic molt.