Difference between SRSW and MRSW safe Boolean registers
You do not know anything about the behavior of the register if read calls overlap.

Perhaps the register is a sensitive electrical component and overlapping reads will blow it up?

Perhaps reading (counter-intuitively) requires changing the state of the register, for instance writing the target memory location which the value must be read in to? If two reads overlap then it might end up writing the value to the wrong location or to only one of the two locations?