Frying whole spices in hot fat appears in nearly every Indian regional cuisine, sometimes as the first step of a dish and sometimes as the last. The two positions do different work.
The technique extracts fat-soluble aromatics
Whole spices hold their aromatic compounds inside cell structures, and many of those compounds dissolve in fat far better than in water.
Hot fat ruptures the structures and pulls the compounds into solution within seconds, which is why the aroma changes so abruptly during the process.
Water cannot do this at boiling temperature, so a spice simmered in a liquid gives up a narrower and less intense set of its aromatics.
At the start it becomes the foundation
When the technique opens a dish, the flavoured fat is the medium everything else is cooked in, so the aromatics distribute through every subsequent addition.
Those compounds then react and mellow over the cooking time, losing sharpness and integrating into a background rather than standing out.
Dishes built this way have depth without any single spice being identifiable, which is the intended result.
At the end it sits on top
Poured over a finished dish, the same preparation contributes aromatics that have had no time to mellow, so they read as distinct and bright.
The fat floats and spreads across the surface, which places the aroma where it will be smelled immediately rather than dispersed through the mass.
This is why a dal that tasted flat transforms when finished this way, without any change to what is underneath.
Some dishes use both positions
A dish can open with one set of spices for foundation and close with another for lift, and the two sets are usually chosen to differ.
The opening tends towards seeds and aromatics that reward long cooking, while the closing tends towards those whose character is lost if cooked further.
Treating them as one step, or using the same spices in both positions, produces a dish that tastes repetitive rather than layered.
The timing is measured in seconds
Each spice has a narrow window between releasing its aroma and burning, and the windows differ, which is why they are added in a set order.
Seeds that pop go first, followed by those that colour quickly, with anything ground or leafy added at the very end off the heat.
Burnt spice cannot be corrected, since the bitter compounds formed are stable, which is why the fat is judged by sight and smell rather than by a timer.