An engineer character can be one of two types of characters. The first of these is a character who creates practical things for use by other characters. If the protagonist comes across a seemingly impossible to pass obstacle an engineer character can help the protagonist to continue on their journey by building something.
The other type of engineer is by far the most common. An engineer character can simply be a background character. In a world full of amazing architecture and engineering feats it is only necessary for there to be some characters hiding out in the background who are designing and creating these places. It makes the world more immersive and realistic. Even if it is impossible to include an engineer character outright you can at least refer to these important worldbuilding characters.
My current project takes place in a university in the world that I created. Because of this it a lot of my characters can be characterized as engineers. It makes sense in this case to have such characters in as major characters in the story.