Why at some raise resolution sometimes and then lower then at other times
In general, I like to work broader at first then progressively finer. Sculpting broad shapes is easier with lower dyntopo resolution (higher detail size value, I like to start at 10). Once I'm happy with my broad shapes, I'll increase dyntopo resolution (lower detail size value, like 5) to accommodate finer shapes. And finally, if there's still more detail to achieve, I'll increase dyntopo resolution futher (lower detail size to 2 or 3) to achieve my finest level of detail.
Sculpting everything from broad to fine using a high resolution (detail size of 5 or less) would generate way more geometry than is necessary. It would bog down your computer and Blender would get slow, even unusable.