Quote from andysmith:
An example:
Say you have $10,000 in an account. Now you put on a 10-pt spread for $1 credit, 10 times (i.e. position size of 10 spreads). Your margin requirement is $10,000. Your credit is $1,000. Your account will be worth $11,000 if the spread expires worthless.
Now say the underlying moves very close to your short strike such that the spread is worth $5 (up from the $1 at the time you sold the spread). You decide to buy back the spread and take a loss. So you buy back all 10 spreads for a debit of $5,000 and close the position. Your account value is now $11,000 - $5,000 = $6,000. You can no longer put on 10x 10-pt spreads for $1 credit... you can put on 6. Had you rolled, you would have been able to roll into another spread with position size of 10.