Solvent cement is not glue simply holds the pieces together. Instead, solvent welding is a chemical process that fuses the pipe and fittings together creating one continuous piece of plastic.
Solvent cementing uses a solvent to soften the surface of the pipe and fittings. When this happens the molecules of the two surfaces are freed to fuse with each other when forced together. The CPVC contained in the solvent cement then fills any gaps between the two surfaces pipe and fitting.
The fused joint becomes the stronger part of the system.