Sometimes they just are. Like my 20-year old corn snake sometimes needs a soak. Snakes dealing with age or illness may need help, when their digestion is no
longer working as well as it once did. A soak can help them re-hydrate, & also encourages them to drink. Not all snakes are in perfect health.
There are exceptions to every rule. But in general, soaking shouldn't be needed as part of the typical shed cycle.