Recipes that call for soaking pulses overnight are managing a transport problem rather than a flavour one. Water has to reach the centre of a dried seed, and heat alone is a slow way to get it there.
What drying leaves behind
A dried pulse has lost most of its water and its starch granules have compacted tightly. The seed coat that protected it on the plant also becomes considerably less permeable as it dries.
Cooking requires water inside the seed so that starch can absorb it and swell. Until that happens, heat only works on the outer layers.
Soaking gives the water hours to move inwards at room temperature, so that heating begins with a seed already hydrated through to its middle.
Why unsoaked pulses cook unevenly
Without soaking, the outside hydrates and softens long before the centre does. The cook is then choosing between a chalky core and an exterior that has collapsed.
Larger pulses such as chickpeas and kidney beans show this most sharply, because the distance from surface to centre is greatest. Split lentils show it least.
That difference explains why split red lentils are routinely cooked straight from the packet while whole beans almost never are.
Why some batches refuse to soften
Old stock is the common culprit. As pulses age on a shelf, changes in the seed coat and in the pectin between cells make them progressively harder to hydrate.
Very hard water contributes as well, since calcium and magnesium ions strengthen the pectin network holding cell walls together. The same batch may behave differently in two kitchens.
Acidic additions have a similar effect, which is why tomato and tamarind are usually held back until the pulse has already softened.
What soaking does beyond speed
Some of the complex sugars that pass undigested into the lower gut dissolve into the soaking water. Discarding that water and rinsing removes a portion of them.
Soaking also begins to reduce phytate levels, compounds that bind minerals in the seed. Sprouting and fermenting continue that process further.
These are gradual chemical changes rather than dramatic ones, and they explain why traditional preparations so often involve a long wet stage before any heat.
Where the pressure cooker changes the calculation
Raising pressure raises the boiling point, so the interior of the pot runs hotter than an open pan can. Hydration and softening both accelerate sharply.
That is why many Indian kitchens treat soaking as optional for pulses cooked under pressure, while still soaking the largest beans out of habit and for texture.
The trade-off is control, since a pressure cooker gives fewer opportunities to check progress before the pulse has gone past the point the cook wanted.