[twawaaay]

When cold air gets heated up the relative humidity gets lower. If you already have very cold winter which tends to be relatively dry -- you should add humidity and this will necessarily cool the air. While this is not strictly necessary, very dry air is not healthy or comfortable. You typically don't need to add humidity when the temperature outside is around freezing but air is very humid.

And conversely, if you live in very hot and humid climate, you need to remove water from incoming air (actually, there is no other way to cool the air). This unfortunately further warms the incoming air and will necessarily lower efficiency of the exchange. Especially because you don't want to remove only minimum humidity -- you probably want to remove enough of it to get to at least 70% for comfort.

So these are extreme climates. In a more moderate climate you can get away with no need to add or remove humidity and you can get as close to 100% efficiency as you want.