The FMS will be set up for the SID, and once engaged with the automatics shortly after take off, it is programmed to fly a far more accurate track than by hand.
That assumes the MAP is exactly accurate prior to commencement of the take-off run. Remember not all FMSs are either GPS fed, or have a R/W Update feature.......
Many years ago now I used to operate into Norwich on the overnight mail from LPL/EMA. Several times I had NDB approaches to R/W09 in very marginal weather (does 09 still have no ILS????). I always flew the backcourse LOC for 27, supported and monitored by the NDB needles. Although flying the NDB approach, it seemed silly to ignore more accurate data that was available, as long as that data was fully supported by the NDB indications.
It's interesting that you ask though - a while back I (a simple controller) mentioned that I thought some crews set up the ILS/loc on departure to use the back beam to assist in staying on the centreline if required (not wishing to open another debate but this would be if the clearance was to climb straight ahead).
R/W33 BHX is straight ahead 2miles, I always if handflying backtrack the 33 LOC. Produces perfect results and is I.M.A.O the best way to achieve accurate 'stright ahead'!
30W