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.
If you use the entirety of your account to support a position, you are flirting with disaster. You have no reserve and cannot kick in the afterburners to get out of Dodge if the need arises. You should have some reserve buying power to get out of trouble if the need arises.

