Good design is unobtrusive
A common problem on council websites is filling up the top of the page with banners containing Mildly Important Information, so many banners the page looks like a local park noticeboard. Service owners are quick to demand yet another banner be put on their pages, or worse the whole site, and slow to agree to those banners being removed when their urgency is no longer relevant.
When you’re piling banner upon banner all over the site, replacing one banner with another banner, and never leaving the site in a banner-free state, those banners become meaningless. They not only get in the way of the user doing the thing they’ve come to the site to do, but they also don’t serve the people asking for the banner in the first place, because if their audience hasn’t seen the information in the banner because they’ve ignored it or simply not seen that the banner talking about their new council tax bill has been replace by a banner talking about cost of living support, then that’s to nobody’s benefit.
A banner, especially a sitewide banner, should only be used in cases of emergency, where a service delivery problem needs highlighting to the user, or where it’s critical every user of the page or site sees it. Remove the banner the moment the point of criticality is over; if the criticality is ongoing for weeks or months, then it’s no longer a critical emergency and the information needs instead to be treated as generally important information rather than critically important information.