This video (at(3:49) says it is a way of freeing processing on the CPU by recording the wave file of the time you set in this box.  At the end of that time the CPU will begin its processing.  Perhaps useful for a full song recording?    I am not sure if this is correct but will try it out.

For What It's Worth I found an explanation in the Cakewalk - Sonar online documentation at this link:  https://www.cakewalk.com/Documentation?product=SONAR&language=3&help=Dialogs2.053.html
 
I copied the description below:
 
Record Pre-allocate File (seconds). When this option is set to a value greater than zero, SONAR will pre allocate the file to be recorded to the size specified (in seconds). This means that the file will not be resized while recording until it reaches the allocated size. The setting has the potential to reduce disk activity while recording and allows for more possible tracks. The valid range is 0–14400 seconds and the default value is 0. A reasonable setting would be 10 minutes (600 seconds) to 30 minutes (1800 seconds).