Smaller files load faster , and that's always better. You can shrink a banner's file size without changing its appearance by using one of the many utilities made for that purpose. Adobe's Image Ready , Macromedia's Fireworks and JASC Software's PaintShopPro all have filters that reduce file size while maintaining image quality. Static banners have a smaller file size than animated banners. That makes them load faster and that can often be more important than a higher clickthough ratio.
Static banners are mostly for other sites to host.
Other people don't want to host a huge, slow loading animated banner because it costs bandwidth and slows their page. To attract the eye . Two layers give you twice the space. If words blink they should blink slowly. Give the surfer enough time to actually read the words .