http://www.railforums.co.uk/showthread.php?t=32811 - there was a thread here a while back.
In essence, in the last decade the service from Sheffield to Birmingham (XC) has doubled, the service from Sheffield to Nottingham (EMT & Northern) has doubled and the service from Sheffield to London (EMT) has doubled, so there's need for additional capacity at Chesterfield (not all stop at Chesterfield, but an additional platform would allow more to stop).
The "semi fast" London - Sheffield service (e.g. the extension of the Derby trains) couldn't stop at Chesterfield on a lot of hours because there were no paths - this will allow more trains to stop there.
AFAIK the only trains to have terminated at Chesterfield in recent years was the Sheffield - Dronfield - Chesterfield "Arriva Trains Northern" service (withdrawn a few years ago - well before the Nottingham "Northern" service) and the EMT Norwich train terminated there too when the Dore/ Totley tunnel was blocked.