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Yes, I dont understand either, what you say makes sense. But here is a quote from the RCOS site.
Secondary Focuser
"Cassegrains have a two mirror, amplified optical system. Mirror spacing and backfocus are directly related to each other. If you change one, it affects the other. Different accessories or cameras will change the backfocus, which will change the required mirror spacing.
Since an Ritchey-Chrétien is part of the Cassegrain family, a secondary mirror focuser is required for optimum optical performance. All of our telescopes are shipped with a secondary focuser as standard equipment."
It doesnt explain why here really, shame. I wouldve thought an external focuser at prime would be easier and cheaper to make, so I guess they thought it necessary.
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