An unusual proportion of Indian sweets start from milk cooked slowly down to a fraction of its volume. That step is not merely traditional; it solves several problems at once.
Reduction concentrates solids and removes water
Milk is overwhelmingly water, with proteins, sugars, fat and minerals dispersed through it in relatively small proportion.
Long simmering drives the water off and leaves those solids behind at high concentration, producing a material with entirely different physical properties.
What began as a pourable liquid becomes something that can be shaped, sliced or rolled, which is the basis for a whole family of sweets.
Removing water removes spoilage
Bacteria need available water to grow, and reducing a food's water content is one of the oldest ways of making it keep.
Combined with the sugar added afterwards, which binds more of the remaining water, the result holds for days without refrigeration.
In a hot climate without cold storage that was not a refinement but a requirement, and it shaped which sweets could exist at all.
The long cook builds flavour that cannot be added
Sustained heat drives reactions between milk sugars and milk proteins, producing the caramelised, nutty character that distinguishes reduced milk from fresh.
Those compounds form over time at moderate heat, so the flavour is a function of the hours rather than of any ingredient in the pot.
Which is why shortcuts using milk powder reach a similar texture but a noticeably thinner flavour, since the reactions were never allowed to run.
Scraping the pan is part of the technique
Solids that dry against the hot sides of the vessel brown faster than the bulk, and scraping them back in distributes that browned material through the mass.
Doing this repeatedly builds flavour unevenly on purpose, creating depth that uniform heating would not produce.
It also prevents scorching, since material left against the metal eventually burns and makes the whole batch bitter beyond recovery.
The endpoint decides the sweet
Stopping while the mixture still pours gives one set of preparations; continuing until it holds a shape gives another; going further gives a firm, sliceable block.
Cooks judge these stages by how the mass behaves against the pan rather than by time, since evaporation rate varies with vessel, heat and quantity.
That judgement is the skill in the sweet, and it is the reason two cooks working from the same instructions produce noticeably different results.