If it could be shaped, it might make for expensive, good heavy armor. But all practical details will depend on how easy the chitin is to get, how easy it is to work with, and what other options exist.
The normal considerations (chitin is heavy, so organisms with chitin armor won't get all that big) can be hand-waved away even without resorting to a fantasy setting. Creatures that live underwater can bear greater weight, for example, and in a high-oxygen biome insect-style respiration is easier and operative over larger distances. Even if we don't get organisms on a scale that would allow for breastplate-sized chitin sections, armorers could achieve a lot with chitin scale and layering.
I don't know how useful it would be as large, solid plates in the first place. Even though it's a strong barrier, there are lots of practical issues. The biggest one I can think of offhand is that it isn't going to be repairable-- you can't melt it down and reforge it, nor weld on a patch, or anything else. You just have to replace the piece. Scale-style armor doesn't suffer that same drawback, as you can replace damaged sections.
Why bother? (alternatively, it has to be worth it)
That isn't meant to be a snarky question. It will only be useful armor if it can provide acceptable protection at a cost in effort, time, and materials that is favorable compared to other options. The alchemy-like method makes that very difficult to estimate (or, conversely, easier to assert, since it's a fantasy technique in the first place!).
As I recall from my biochemistry courses, chitin is a polysaccharide (a sugar, though certainly not a table sugar) with a particularly tight angle of rotation that makes it very resistant to solvents, like water. You're not going to be able to work chitin at all like you would work metal or leather, so the alchemy which makes it possible is whatever you imagine it to be and it works however you imagine it should work.
With that in mind, secondary effects of the giant-chitin become important. If it makes the chitin easier to work with, might it cause the armor to lose its most important properties if sprayed onto combatants during a battle? Even if it can be worked, how much time and effort does it take to make a set of chitin armor versus other options, such as chainmail, full plate, wood armor, boiled leather, or quilted armor? If the armor is so good, how are people killing the insects already covered in it, and why would a person wearing it fare better?
tl;dr: If it can be shaped to be appropriate armor for a human, it's probably going to be better than being unarmored in many situations. How useful it would be is a function of how much better it performs than the next-best option available to the people that would be making it, and how much more or less effort it requires than the next-best option. And even then, practical concerns might make plate less desirable than other choices, like scale.
This is all fantasy, so you can definitely contrive a situation in which chitin armor is a practical choice and a situation in which it is the best choice. But a situation like that will have less to do with the chitin than it has to do with all of the non-chitin options available.