Engineering Design Jigs & Fixtures
To hold components where you want them you need a jig or fixture 
No matter if your product is assembled via automated plant or by hand, at some point during assembly parts need to be held to achieve positive location so that assembly may take place accurately and efficiently. Some parts such as those made from elastomers, are very difficult to handle at the assembly stage becasue of their deformation properties and need to be rigidly held in their non-deformed state (as tooled). Jigs and fixtures may be bespoke or modular designs (especially for automated assembly & plant) so that they may be quickly changed over to alter the function of the automated plant i.e the production line can handle different parts for automated assembly. The same may be true for hand assembly, where fixtures can be altered to change function.