verb + の + ですか　？（what is the の doing?)
I am trying to understand why this hiragana is used to ask certain types of questions.

Like this:

彼らは何を見ているのですか

あなたはどうして笑っているのですか

友達はどこに行くのですか

is there a difference between this and saying something like:

友達はどこに行きますか

Thank you :D

Question

verb + の + ですか　？（what is the の doing?)
I am trying to understand why this hiragana is used to ask certain types of questions.

Like this:

彼らは何を見ているのですか

あなたはどうして笑っているのですか

友達はどこに行くのですか

is there a difference between this and saying something like:

友達はどこに行きますか

Thank you :D

Answer

It implies that you are asking for a reason. 何やってるんですか？For example. It opens up for the other person to explain. Without の/ん/なん　it's just a statement. That is the essence of it.

Evden çıkmadan dil öğrenme fırsatını kaçırmayın. Deneyimli dil eğitmenlerimizden oluşan seçkimize göz atın ve ilk dersinize şimdi kaydolun!