Basically its for the interpolating oscillators. Such as; 

http://www.csounds.com/manual/html/oscili.html

There may be a clearer explanation elsewhere in the 
manual, 

This below is a section copied from here; 

http://www.csounds.com/manual/html/f.html

>>
For arrays whose length is a power of 2, space allocation always provides for 
2n points plus an additional guard point. The guard point value, used during 
interpolated lookup, can be automatically set to reflect the table's purpose: 
If size is an exact power of 2, the guard point will be a copy of the first 
point; this is appropriate for interpolated wrap-around lookup as in oscili, 
etc., and should even be used for non-interpolating oscil for safe 
consistency. If size is set to 2 n + 1, the guard point value automatically 
extends the contour of table values; this is appropriate for single-scan 
functions such in envplx, oscil1, oscil1i, etc.
>>

On Tuesday 12 January 2010 10:43 am, Emerson Aagaard wrote:
> I was just curious (and I was sure someone could answer this question in no
> time on here) what the significance is of adding one to the size of an
> f-table?
> I've looked through the manual and have been reading the Csound book for
> some time now, and I don't remember ever seeing clear explanation for this?
>
> Thanks much,
> Emerson
>
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