tl;dr- Magic is weakened when observed
The Blessed are empowered by the mechanisms of the world to serve the interests of its creators. But since the creators have different interests and their opinions can change, the magic of the Blessed can strengthen or wane according to their actions.
The Blessed don't fully understand the situation, but they generally appreciate the fact that showing off their powers – or, worse, using their powers in a disruptive way – tends to diminish their abilities. So, the Blessed tend to be reserved in both the disclosure and use of their powers.
Lore: The demiurges indirectly grant the Blessed power
At the dawn of Creation, the demiurges had come together to build the world. As sapient beings, the demiurges had goals in mind – we're not exactly sure what these deitistic figures wanted from the world, however it seems that they designed the world to serve their various agendas.
For the most part, the world is largely deterministic, playing out like a physics simulation. Rocks fall due to gravity, the sun warms by day, the snow melts when heated, and so forth.
In principle, an omniscient God could've designed the world to play out exactly as they intended, tweaking every little bit of Creation as They would see it. However, the demiurges weren't quite so limitless; they couldn't foresee every moment of the future from the beginning, so to ensure the fulfillment of their agendas, they put in automatic course-correction mechanisms that would help guide the world as they'd want it to be guided.
We don't know of many of these mechanisms. In fact, many of the demiurges had no concern for life as we know it, but rather cared about how water flows in the oceans, or how the winds blow through the hills, or how high the trees could grow in the forest; and so, they laid down mechanisms to ensure their inscrutable designs, largely invisible and beyond our appreciation.
Other demiurges had concerns that we might better comprehend, such as a will to see humans live in certain ways or to enforce certain ideals; or, in some cases, demiurges who had no real concern for humans intended to exploit them as tools to help maintain their true concerns, e.g. Druids who are empowered to protect nature. But whatever their motivations, many of these demiurges ended up creating the Spiritual Observers who observe the world and try to pursue their particular goals by providing humans with powers; we know them as the Blessed.
This leaves the Blessed in a strange position, as their powers aren't absolutely guaranteed like the more deterministic parts of physics, but rather are tentatively allowed to override normal physics in service to the interests of the forces of Creation. The Blessed may not know or even be capable of understanding any of this, but they nonetheless serve the interests of those that have empowered them.
For example, a Blessed who can heal seems to grow more powerful as they help others or weaker if they cause harm, even if they do so without using their magic! Likewise, a Blessed who has powerful Fire magic may grow stronger as they use it, unless they do so in a way that harms others or nature.
Behind the scenes, this is because the mechanisms installed by the demiurges are balancing their competing interests. The healer or fire mage who uses their power is often serving their sponsor's desire by doing so, strengthening the trust and gaining more power for it. However the healer who hurts others loses their sponsor's trust and thus their power. Likewise, the fire mage who burns down a forest may not upset the interests of their sponsor, but they do upset the interests of the demiurges who valued nature and therefore contest the fire mage's continued privilege to override physics.
This leaves the Blessed in a fairly political position, where their powers grow or wane with the support or objection of the mechanisms that allow for magic to override physics. A Blessed's closest political ally is typically their sponsor, i.e. the mechanism that gave them their power in the first place. And while it's helpful to gain support from other mechanisms, more often than not, other mechanisms are more likely to find fault with their magical privileges, weakening the Blessed. So to avoid power loss, Blessed learn that they should avoid unnecessary displays of power and even disclosure of what powers they have that a force might object to.
Of course, all of this is behind-the-scenes knowledge. For the most part, the Blessed don't know why showing off their powers tends to diminish their abilities, but there's still a general awareness that their gifts can be lost, and that making others aware of their gifts significantly contributes to such loss.
Mechanics: Balancing interests make for an easy fictional world
This description of the Blessed and the larger world is heavily based in the very forces of Creation changing their estimates to seek some goal. The non-omniscience of the creators prevents the world from being boring, but the idea that they're continually trying to nudge the world according to their various agendas creates sort of a dynamic equilibrium that helps keep everything in-check.
For the Blessed in particular, a backdrop like this can help explain why very powerful individuals would be reluctant and even unable to exploit their powers in ways that it'd seem like they otherwise might. But there's also variation in the Blessed's willingness to reveal/misuse their powers, as they may vary in their perceptions of the situation and their willingness to risk negative consequences.