The Messenger of Allah ﷺ used to give to some of the detachments he sent out (something extra) for themselves in particular apart from the division made to the whole army. The fifth is necessary in all that.
أَنَّ رَسُولَ اللَّهِ ﷺ قَدْ كَانَ يُنَفِّلُ بَعْضَ مَنْ يَبْعَثُ مِنَ السَّرَايَا لأَنْفُسِهِمْ خَاصَّةً النَّفْلَ سِوَى قَسْمِ عَامَّةِ الْجَيْشِ وَالْخُمُسُ فِي ذَلِكَ وَاجِبٌ كُلُّهُ