It seems to me that these 3 paragraphs summarise the "why" within your question, AoG.
"The Ministry of Defence said specialist advisers from the Brigade Advisory Group, made up of the 4th Battalion The Rifles, provided support to the UK-mentored 3/215 Brigade for the eight-day mission, "in line" with their current advisory role.
In a statement, it said: "In support of the Afghan forces who now have the security lead across the country, UK personnel do on occasion operate outside the usual UK area of operations in central Helmand in an advisory capacity.
"These out-of-area operations have been a long-standing element of the UK mission in Afghanistan and are completely in line with our current role of providing training, advice and assistance to the Afghan National Security Forces."