On the outside, they're representative of darkness and great power, wrath, and capacity for action, often shown in striking visage.:
Its in this space where there are similarities to what Guru Padmasambhava is credited for doing in Tibet.
Where it differs, however, is that these wrathful and malicious spirits turned to protection weren't removed of their bloodthirstiness. Instead, they were accepted and conjured as such, with full understanding of their nature and the recognition that this too was dharma and an expression of nondual nature.
The trouble is that this was later intentionally sanitized by the Japanese government during a period known as the Shinbutsu Bunri. During this time, the Japanese imperial government was in full scale colonialism mode, and they essentially killed, disrobed, or otherwise converted Buddhist practitioners en masse. This was a violent show against the priests and nuns who had been these bridges between indigenous Shinto practices and Buddhism that'd come from China with Kukai.
When you start looking at this in historical retrospect, and you're confronted with images like this, it changes the way you look at the circumstance itself.:
So these sanitized ideas and ideals around Japanese Buddhism and Vajrayana in particular are colonialist state ideals, and it would serve us collectively to view and experience these spirits as potentially violent, wrathful and STILL helpful.
Food for thought.
-Kit & Vestige, Tea's system