It's been a while since I was involved in the PMT side of things, so I don't know how things have changed with the main airport having moved. Previously though the choppers were only very rarely used to transport from King's House to the airport or anywhere within reasonable distance of Durban, unless there was a problem on the route - it's quicker and cheaper to drive otherwise. The choppers collect the President or DP from King's House and from there go to whatever venue they're visiting, say PMB, Nkandla, or Ladysmith.
And in terms of why do we have all this expensive military equipment? Ask the people on that cruise liner that the 15 Sqdrn guys rescued how much of a wasted expense they are. Ask the people we rescued from the floods in Mozambique, or the guys from the burning apartment block last year, or the hikers they pulled out of the Berg this past weekend.
A military is a multi-purpose tool, it's not just used for fighting wars. It also pumps a lot of money into the economy - 60% of our budget is salaries, and that goes straight back into the economy, never mind all the money we spend on services which we are not allowed to do ourselves. And frankly, you are paying peanuts for the Defence Force, 1.3c of every rand of tax you pay. We are so underfunded it's a joke. Think of it like an insurance policy - you hate having to spend the money on it every month, but you're well up **** creek without a paddle if you don't have it the day somebody steals or smashes your car.