Writs usually are considered to be extraordinary remedies, meaning they are permitted only when the defendant has no other adequate remedy, such as an appeal. In other words, a defendant may take a writ to contest a point that the defendant is not entitled to raise on appeal. As a general rule, this applies to issues that are not apparent in the record of the case itself (such as when an attorney fails to investigate a possible defense).
Any one of the following reasons, for example, may prohibit an appeal (and justify a writ):
Proof & Defenses in Criminal Cases
Getting a Lawyer for your Criminal Case
Steps in a Criminal Defense Case
Arraignment: Your First Court Appearance
Plea Bargains (Deals) in a Criminal Case
Legal Elements of Common Crimes
Expungement & Criminal Records
Should I just plead guilty and avoid a trial?
Is the public defender a real lawyer?
Can I change defense lawyers after I've hired one?
How long after arrest do I find out what the charges are?
Does it matter whether a suspect is given the Miranda warning?