Why is the address (URI) of a WordPress blog static

I wonder why the address (URI) of a WordPress blog is static and must be specified explicitly under “Options”.

Here is my problem:
I’ve set up a WP blog on my personal notebook. After that, I registered at DynDNS, because I only get dynamic IP addresses from my internet connection provider, and I want my blog to be accessible from the internet by a static host name.

Let’s say that my blog has the URI “https://xyz.dyndns.org/wordpress”. I specified this URI under “Options -> General Options”. If my internet connection is up, my blog is accessible from the internet and local, too. But if my internet connection is down, I cannot access the blog locally, although I made an respective entry into my /etc/hosts (under Windows XP Pro). If I try to access my blog via “https://localhost/wordpress” I can see the frontpage of the blog but cannot access any links or categories, which seems to be caused by the explicitly specified URI under “Options -> General Options”, as every link points to “xyz.dyndns.org”.

My question:
Is it really necessary to explicitly specify the URI under “Options -> General Options”? If yes, why does WordPress need it, when every HTTP server doesn’t need it? Are there any ways to solve this problem?

Cheers
D.