What is the true meaning of「ておく」?

Question

I'm very confused about the meaning of「Vておく」after watching this video.

I usually interpret「Vておく」as "to do something in advance/do in preparation for future." The speaker in the video mentioned that it is not always true. He said that「Vておく」actually means to do something and put the results in effect. I'm not sure how it's different from doing something in advance. He provided some examples where「Vておく」doesn't mean to do something in advance.

For example,

子供を部屋に閉じ込めておくのはひどい。

It's cruel to lock a child in a room.

He mentioned that the「Vておく」in this sentence doesn't mean do something in advance but do something and leave its effect in place. If I think「Vておく」in terms of future preparation, I can understand this sentence like this

It's cruel to lock a child in a room (to torture the child or something).

Which of the two interpretations above correct? You can see the second interpretation directly contradicts the fact that「Vておく」don't always mean to do something in advance.

Another example,

時々赤ちゃんに泣かせておいてもいい。

It's alright to let a baby cry sometimes.

Once again, he mentioned this「Vておく」doesn't mean to cry in advance. But, I can interpret it in another way. In terms of future preparation, this sentence can also be rendered as

It's alright to let a baby cry sometimes (so that the baby can calm down).

Should I always interpret「Vておく」as "to something in advance"? More precisely, does「Vておく」always carry the nuance of preparation for future?

Answer 1

There are two meanings for ～ておく、the first of which you have already noted is do something in preparation for the next action/event.

You can see, for example, this link for example sentences.

The second, also in the link above, is to do something and then leave it in the state. Also recall the common verb ほっとく＝放っておく, which clearly is not the first usage.

Your interpretations are fine, but you are forced to make an assumption in order to do so, so it is probably simpler to just understand the example sentence in your question as the second usage (as your linked video recommends)