DesignSpark PCB searches for a component outline by default and uses this to check the Design Rules.
If it cannot find an outline it uses the silk screen shape as its backup.
The issue discussed above is for a TH component and if there is no outline defined the DRC uses the silkscreen which will also be projected 'through the board' as if the component is on both sides! This not normally an issue with TH components until you place components on the lower side of the PCB under the TH component.
So, the uses of the Component Outline are;
1. allow the component shape to be allocated to one side of the PCB
2. allow different outlines on each side of the PCB for DRC purposes
3. If the outline is different from the silk screen shape.