To begin with, recall that Kantian deontology is a form of deontology that places absolute moral value in the agent's intent. The way you figure out if an intent is good or not is by seeing if it follows the categorical imperative.
One way to see that intentions are more important than circumstances is through a thought experiment.
Imagine two people, each driving in their own car. One respects human life, but accidentally hits a pedestrian, killing them. The other is evil and intentionally murders someone with their car.
In both cases someone died. But the intent of each driver was different. For this reason, the one that didn’t intend to kill anyone isn’t morally blameworthy, whereas the person that did intend to kill someone is morally blameworthy.
If you agree that these two cases do differ morally, then you accept it is intentions that matter, not circumstances. This gives Kantian deontology a strong advantage over utilitarianism. Since the utility is the same in both cases, utilitarianism wouldn’t distinguish them like the Kantian deontologist does.
There are many similar cases that prove this point.
The outcome is the same, but only the first person was moral because of their good intention. The other person did it purely for their own benefit, and so doesn’t have moral worth.
One of the reasons Kantian deontology is attractive for many people is because it backs up an important intuition most of us have. This is the intuition that people have value beyond being used as mere tools.
For most people, there are just some things more important than happiness. This is genuine freedom or the ability to be a self-determined agent. Kant’s formulation of humanity grasps this idea. It says that our humanity is more valuable than anything else and so should be respected at all times.
Another aspect of Kantian deontology that makes it attractive is the fact that it fits with many of our ideas about what morality should be like.
Although many of us feel strongly about these things, we can’t always say why we think they’re so important. A great advantage of Kantian deontology is that it offers an explanation for these deeply held beliefs. This is especially important in philosophy since philosophers argue that our beliefs should be justified.