A plan might be compared to mapping out a detailed path, whereas a purpose may involve having in mind a goal but not necessarily one specific manner of reaching it.
مِنْ بَعْضِ ٱلنَّوَاحِي ،‏ يُظْهِرُ وَضْعُ هذَيْنِ ٱلرَّجُلَيْنِ ٱلْفَرْقَ بَيْنَ ٱلْخُطَّةِ وَٱلْقَصْدِ .‏