What parts of the 787 are produced with 3-D printing (besides air ducts and hinges)?
Another example of printed parts is electronics covers.

It wasn't necessarily "essential" to use 3D printing, but economical or beneficial versus other choices. For aircraft parts, 3D printing has a few advantages over more traditional methods of manufacturing:


Much less delay for manufacturing. A new part can be designed and
then 3D printed very quickly. Traditional methods need much more time
to create tooling for each new part. If you are delaying a product
worth millions, time is important, and the faster method will have a
big advantage.
There can also be less cost in manufacturing. The cost of initial
tooling for a part only pays off in the long run if many parts are
produced. A 3D printer can potentially make many different parts,
with minimal added expense to make a new design.
The design can also be made more efficient. 3D printing can create
shapes and designs that are not practical with traditional methods.
This can reduce the weight and complexity of parts.