Some SharePoint objects needs an instance to work perfectly, one example is here;
Sample 2 works however the Sample 1 doesn't :)
Sample1
SPSite site = new SPSite("http://server");
SPWeb web = site.OpenWeb();
web.RootFolder.WelcomePage=”default1.aspx”;
web.RootFolder.Update();
Sample 2
SPSite site = new SPSite("http://server");
SPWeb web = site.OpenWeb();
SPFolder folder = web.RootFolder;
folder.WelcomePage = "default1.aspx";
folder.Update();