"Post-marinating"? Is it a real term or do my taste buds deceive me? Two nights ago I cooked some chicken on my George Foreman grill, just plain (a bit of non-stick spray but that's it). Then I chopped it up and put it in rice and stuck it in a tupperware for lunch today. I then rummaged my fridge for something to add flavor. Well I had some soy sauce and some teriyaki marinade, so I poured some of both in the tupperware, making sure to mainly coat the chicken with them.

That was two days ago; this morning when I got it out of the fridge before leaving for work, the sauce seemed to have soaked into the chicken, so I wondered if it's any different from marinating before cooking. And just like I thought, I just finished my delicious chicken and rice and if I didn't know better I would swear the chicken was marinated before cooking.

Is there any difference between marinating before or after cooking? I mean obviously if you cook, then marinate, then eat it right away it won't be soaked into the meat at all. But something like this where I cooked it and THEN marinated it, is there any difference? Does the meat marinate just the same, and is there any reason I shouldn't continue to do it this way rather than preparing better next time and making sure to marinate my chicken beforehand as is generally done?

Sorry if I sound completely naive, I am indeed very new to cooking (being a college student and all) and I've just never heard of post-marinating something, so I wonder if it's commonly done.
There are several reasons why you should marinate before cooking:


Many marinades contain raw ingredients that should be cooked along with the food being marinated, such as garlic or ginger.  In some cases this may actually be a health hazard (raw garlic can harbor botulism), in other cases you'll simply end up with an undesirable pungent flavour.
Many marinades also use some amount of acid, which helps to kick-start the process of breaking down the connective tissue and tenderizing the meat.  This is an especially big deal for chicken breast meat and tougher cuts of beef or pork, because most cooking methods (except for slow-cooking) don't do much of this and tend to produce a tougher, chewier end result.  Once the meat has already been cooked, using an acid-based marinade won't give you much more tenderization; you need the combination of acid and heat.
Finally, many spices release most of their flavour when heated (cooked).  Examples are cinnamon, saffron, star anise, and various types of hot pepper or chili powder.  If you marinate after cooking, you won't get much flavour from these.


Of course if you are using a very simple marinade such as soy sauce and are marinating only for flavour purposes, then it might not matter much, but that's the exception rather than the rule.  Marinating afterward can certainly improve the taste, but most of the time you'll end up with better results by marinating beforehand.