Several reasons to get a second opinion:

1)     To answer a code issue brought forward by the building official

2)   To get an engineer to review and sign off on work done without inspection when
       required by the building official or lien holder.

3)     To get an alternative to the architect or project engineer of record.  Usually done when cost and time are undesirable.  Sometimes to confirm architect or project engineer of record course of action.

      4)   Whenever you think you need a second opinion or alternative