What are the reasons for rebuild and redeployment libraries?

Question

It's difficult to understand the meaning of the question from the topic's name.

Here is what I mean.

I'm watching Uncle Bob's Clean Code Episodes. In many episodes where he is talking about SOLID principles he mentions the problem of rebuild and redeployment caused by code changing. He says that bad architecture causes frequent rebuilds and redeployments.

But I have a couple of questions:

1. Why frequent rebuilds and redeployments is bad?

2. Can you enumerate causes of rebuilds and redeployments of dll's?

3. Is there any difference between different languages according to the second question?

4. What causes rebuild and redeployment of dll's in C#?

The second question (and the most common) is based on dll's coupling. What should I change in one of two coupled dll's so then I have to rebuild and redeploy the other?