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DDR is based on dual edge signal: ie both rising and falling edge...when you are writing into the RAM you controller provides a clock to DRAM. but when you are reading from the RAM the strobe works as the clock for the byte that is read in. The strobe is transmitted with the data as the dram doesn't send a clock back to the controller. then the strobe edge is changed by 1/4th of the cycle and used to sample the data.
For DDR interface where data transaction happens at both the edge which makes things faster. Strobe is a DDR necessity