Coolant gets used up... additives get depleted... pH changes. If you wait until it looks like it needs replacing, you're too late, the damage has started.

I like to flush because I feel the turbulence will sweep up and carry away any contaminants, plus it gets all the old fluid out of there... kinda like letting oil drip out for another 15 mintes after draining the engine oil.

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95 E320 Cabriolet, 169K