Currently any file created in a normal tiered volume(where there is sufficient space on cold and hot tier), gets created on hot tier.
But there can be usecases just like hospitals, where they create and save files to rest, till recalled due to an event. In such cases it would be good to use the slower disks directly than create the file on hot tier and then move them to cold, when in such environments it is highly improbable to immediately recall a file created just few seconds or minutes back.
This will help save the life of ssd with unncessary writes and save on n/w bandwidth by not having files to be moved from hot to cold once the file becomes unaccessed for long.

Having hot tier as default makes sense in significant usecases where the newly created files will be reaccessed again.
But having hot tier as default create place will not be much useful for files in work environments we know the files are not re-accessed once created