The room next door

The room next door
 

The Spanish title is La habitación de al lado,

Why is deused? If I were to translate the title, not knowing the correct grammar, I would have left out the de.

Both «de al lado» and «al lado» can be translated as 'next door', but grammatically they are different.
When used as an adjective, as it was in the title you were quoting, the correct form is «de al lado».
However, if a verb is involved, and 'next door' describes 'where', you are using an adverb ('down/up/along the way/next encountered'). This is when «al lado» can be translated back into English as 'next door'.