Ok, let's try to be more clear about this ..
Structure size is say 1000 Bytes, when writing the entire structure the
write time would therefore be 9mSec * 1000 = 9 Seconds.
Typically in our situation, parts of the structure change but other
parts stay the same (this is configuration data for a PLC-like device).
When downloading the entire structure from a PC, quite a bit of eeprom
memory already contains the correct values.
So instead of waiting 9 Seconds for a full structure update (and another
9 seconds for a backup structure), we first read before write each byte,
typically saving us quite a lot of time.