Caching GRV lets you read the data at an older version (i.e., the data that is a little bit stale).
If your transaction has only read operation and can tolerate the staleness, then you can read data up to 5 seconds ago.
If your transaction has both read and write operation, then your transaction may have higher possibility of conflict with other transactions operating on the same data. But the safety property – only one transaction can succeeds and only one client read and write the data – is still valid.