This feature is useful when using more than one program for a variety of tasks. For instance a part may be designed in 3d parametrics, but then a 2d print is needed to produce it. Alternately a simple 2d part can be made using a program like AutoCAD, but a 3d model may be needed to facilitate an assembly view.
Many programs, including Inventor and Solidworks, include integrated support to make 2d "prints" from the regular native 3d parametric model. However, personally I find these abilities to be a shortcut and sometimes you loose the flexibility that comes with a dedicated 2d program such as AutoCAD or similar. Myself I design almost exclusively in Inventor, but 99% of my actual 2d prints are made using replicas in AutoCAD. If the part is simple then I'll make a "wizard" print within Inventor, but I don't do this very often.
Exporting Inventor files to 2d:
This is primarily focused on creating a DWG or DXF file from your Inventor part files. You can't do this all in one step, but it can be very useful when working with part features individually.
To start, I use this 3d model as a demo. It's composed of a main revolution then secondary subtractions are made from it. The secondary features are all simple (merely circles, radiused rectangles, etc) so there's no need to "export" those sketches. However, the main revolution profile is intricate and already dimensioned, so this would have to be recreated line-by-line in AutoCAD.
I want to create a 2d model for this part so the first step is to export the sketch into a 2d format (DXF in this case) all you need do is locate the appropriate sketch in the feature panel, right click, and select Export Face As... (the name is ambiguous since you're exporting a sketch rather than a surface as implied).
A new file called "housing_outline" was created in the above directory, and can be opened using AutoCAD or a DXF/DWG file viewer.
In this case, if I wanted to use the sketch to edit and create dimensions, I would open it in AutoCAD.
The imported geometry is a polyline (a combined shape composed of multiple smaller line segments) so you must explode it before it can be edited. This is located in the modify toolbar, or by typing explode into the command line.
Once exploded, you can now assign layers to the lines, add construction geometry and dimensions, etc etc. You can even make a 3d wireframe or old-school 3d solid within AutoCAD if you wanted. Below is a screenshot taken after a few arbitrary dimensions were placed.
Importing AutoCAD files to Inventor:
This process is even easier than above since you need not create go-between files for the import/export process. For demonstration I'll take this old print I made of a frame model a few years back...
It's pointless to import everything shown, so you'll need to choose which specific lines will be the most useful in the new 3d model. In this case I'll take the external view of the frame's profile.
Highlight everything you want to take over, and copy it to the clipboard.
Open Inventor and create a new 2d sketch on whichever plane you wish to use for the improted lines. When you hit paste the system might stall for a second but when it recovers it will prompt you with a dashed-line outline of your imported shapes. It's difficult to control where exactly they will get placed so you will likely have to choose an arbitrary spot within the window (which is what I do).
Click again and the copied shapes will appear! In this case there are more lines than I need to create the proper extrusion, but I'll keep the superfluous lines to use for reference later.
In the third picture above, I enabled visibility on the 2d sketch used to create the solid extrusion. You can see how this will become useful in additional features made to this extruded profile, such as the spine fillets, through-holes, trigger guard shapes, etc. You can create reference geometry in new sketches based on this original sketch instead of importing more features from AutoCAD (which would do the exact same thing in the end).
Importing AutoCAD files to Solidworks:
This process is more convoluted than with Inventor, but the results are equally as useful. To import 2d parts into SW, you must use the import wizard and don't need to actually have AutoCAD on the computer (unless the DWG file is locked).
For an example part I'll take my 2d prints of a solenoid manifold I used some time ago. Within Solidworks, use the Open window as normal, however you must change the filetype selection to the appropriate extension (DWG, DXF, etc).
Select the AutoCAD file and the Import Wizard will appear. It gives you selections toward how the AutoCAD file will be imported, such as in a new sketch or a whole new component. Another option is the layers you want to import. Obviously you don't need to import any dimensions, section lines, etc.
You won't be able to import one portion of the drawing unless it consists of its own layer, so you'll end up importing the whole 2d file. That's okay since you can use the sketch for reference (at worst it'll add a few bytes to your filesize).
Anyway, process the import and the new 2d sketch will appear. Also the 2D to 3D toolbar appears on its own, just close it if you won't want it.
Pick one of the shapes to act as your base feature, and do your thing.
As with Inventor, you can choose to