There isn't really anything novel to me in this letter. Seneca makes two points about fear and his musings are based on his experience while travelling, which forced him to go through a very dark and dusty tunnel.
The first point: even a person whom "fortune has lost her control" (in other words, someone who has make exceptional Stoic progress of being indifferent to circumstances), will still experience involuntary movements. Seneca writes:
For there are certain emotions, my dear Lucilius, which no courage can avoid; nature reminds courage how perishable a thing it is. And so he will contract his brow when the prospect is forbidding, will shudder at sudden apparitions, and will become dizzy when he stands at the edge of a high precipice and looks down. This is not fear; it is a natural feeling which reason cannot rout.
This involuntary movement or emotion is described by Aulus Gellius' in The Attic Nights where a Stoic philosopher experiences the same initial emotions stemming from a raging storm. Many Stoic authors have referred to this experience. Donald Robertson does a good job explaining it in his essay "Epictetus: The Stoic in a Storm at Sea."
The second point from this letter: our fears can be irrational if the end result is the same. We should overcome the fear of the result, not necessarily the thing that would cause the result. Seneca endured the dark and dusty tunnel. And as he saw the light at the end of the tunnel, he observed his own emotions and thoughts.
Then at the first glimpse of restored daylight my good spirits returned without forethought or command. And I began to muse and think how foolish we are to fear certain objects to a greater or less degree, since all of them end in the same way. For what difference does it make whether a watchtower or a mountain crashes down upon us? No difference at all, you will find. Nevertheless, there will be some men who fear the latter mishap to a greater degree, though both accidents are equally deadly; so true it is that fear looks not to the effect, but to the cause of the effect.
He then gets into the immortality of the soul and whether it remains immortal if the body gets crushed. It's a bit of an odd ending and I didn't get much out of it.