I've read some answers say that you should put things like cloves, black pepper, bay leaves, etc. at the end of cooking a curry, as spices will loose their properties. I notice that Bangladeshi cuisine always uses these items at the very beginning of cooking. Why so? I'm imagining its been done like this for a long time. What do they know that we don't? Perhaps there is some other goal?
Just like many other ingredients, when spices are added to a dish is a function of what effect you want them to have on the final dish. If you want deep, well-integrated flavors, or even undertones, you add them early so the flavors meld into the product. If you want a more pronounced flavor or an aroma, they are added at or near the end of cooking. In addition, you can certainly do both, as the flavor and aroma can be enhanced by this double addition (at the beginning and at the end). When these aromatic spices (and often alliums) are used, like in curries, they are heated in oil or ghee. The oil is flavored, and this helps to carry the flavor of the aromatics throughout the product. Spices and herbs also contain many volatile compounds, which will be lost with heat and time. In this case, a last minute addition, just before serving, is the best approach.
"What do they know that we don't?" Well, "they" know the final result that they are trying to achieve, and how ingredients behave at various points in the cooking process. The end goal is a well-integrated dish.
It really depends on the specific spice/herb, and on how strong you want it to be in the dish.
Some aromatics are destroyed by prolonged heating, while others can't be tasted unless left to steep in the dish as it cooks.
Cumin, mustard seed, coriander seed and others are at their most powerful when mixed into oil/fat at high temperature, usually in the beginning of the dish with the onions.
On the other hand, turmeric or nutmeg would lose their flavour at high heat, and usually get added near the end, or at least after the water is added.
It can be even more complex. For example, salt added at the beginning will take water out of ingredients and into the sauce. Salt added near the end will just make the dish saltier.
There are also things like bay leaves or tea leaves, where the flavour depends on how long they have been cooking.
I've always been told a very simple way of knowing:
Dry seasonings go in at the start.
Fresh seasonings go in at the end.
This applies mostly to herbs, but works for pretty much anything.