The verdict that water (bath) is necessary when there is emission given by the people (in the early days of Islam) was due to the concession granted by the Messenger of Allah in the beginning of Islam. He then commanded to take a bath (in such a case). Abu Dawud said : By Abu Ghassan is meant Muhammad b. mutarrif. (Using translation from Abū Dāʾūd 215)
أَنَّ الْمَاءَ مِنَ الْمَاءِ كَانَتْ رُخْصَةً تَرَخَصَّهَا رَسُولُ اللهِ ﷺ فِي أَوَّلِ الزَّمَانِ ثُمَّ أَمَرَنَا بِالِاغْتِسَالِ بَعْدُ