Without CRC there is no error detection on the SPI bus. Read/write data errors will not be detected if the SPI signals are marginal. Here is an example of what can happen:
http://arduino.cc/forum/index.php/topic,97456.0.html
Enabling CRC will cause the SD card to check the CRC on commands and write data. The SdFat library will check the CRC on read data. This dramatically reduces the possibility of undetected errors.
See this for more info Cyclic redundancy check - Wikipedia.