Readunique and cleanunique transactions in ACE protocol

In case of readunique transaction cache line is copied into the initiating master's cache(whether it is clean or dirty) and invalidated in snooped master's cache and then store operaation is performed in initiating master's cache line.

In case of cleanunique transaction cache line is copied to main memory if it is dirty(and invalidate)  and then  store operation performed in initiating master's cache.

Why do we need extra write to memory in case of cleanunique transaction? 

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