South East Asia is a very humid part of the world. I think this is pretty well known even for people that have never been here. They unfortunately do not alter the way that they build to accommodate this and often or at least most of the time in places I have been to, this results in bad things happening with the damp such as your clothes getting all stanked in the closets and the general absence of any sort of storage because anything you put in there is going to be subjected to mold.
I am not really up to speed on mold and what it can do to you, but I know that it isn't good. I know a few people that have rather severe allergic reactions to it and am thankful that this person is not me. I don't know what it does to me but I do know that I was having strange sleep situations where I just didn't feel right even if I slept for a very long time.
Then I bought a dehumidifier and things have changed a lot since then.
The other day though it occurred to me that all the times I have seen these machines in my life they have been in areas where ventilation was not possible such as a basement in a childhood home. The rooms of my apartment do have windows in all of them, so I actually can get ventilation if I need it.
But herein lies a conundrum that I wasn't aware of until I was presented with possessing this machine: If I open the windows, am I not actually reintroducing humidity to the room in question? I thought initially surely not but as it turns out the answer is actually yes. So now the room that I sleep in that contains the dehumidifier I just keep the window closed all the time now. I"m not going to go super crazy and keep all the windows closed all the time but because of the noise of this city I rarely open the windows anyway. "fresh air" is one of those things that doesn't really exist in this city anyway because there are a million people living in a place where there should probably be a maximum of half that many.
So if you are ever faced with this issue, which I suppose is unlikely unless you live in an area similar to my own and everything is made with poorly put together cement like is the norm here in Da Nang.
Now I think I might get involved in getting an air purifier next because I am on a roll as far as air quality is concerned ever since I started having strange respiratory issues out of nowhere.
Particularly 
I have a very specific set of skills and none of them apply to things like this so i'm basically totally in the dark here. But I guess it makes sense that if you are trying to eliminate humidity from an area that opening a window that is definitely going to allow humid air in does make a lot of sense.