Why are dynamic base map services offered in addition to tiled services?

File sizes to store tiles increase 4x with each zoom scale when building a cache for a tiled base map service.  The storage size required to cache data at scales larger than 1:18,000 is greater than USGS can currently store.  For example, just to store data down to 1:18,000 requires a cache of over 39 million tiles. Thus, high-performing dynamic services were created to support scales of 1:9,000 and larger for various composite base maps.