Just a note on the function of the praetor.  The praetores were not judges.  They were magistrates tasked with the administration of justice, in the sense that (and this was so from the time the formulary process was used) they would assist the litigants in formulating the claim, whereafter it would be reduced to writing (the formula) and proceed to trial before a judge appointed in the formula by agreement between the parties.  In South Africa (and I would imagine also the UK and most countries that follow the Anglo-Saxon tradition) pleadings are usually ended with a prayer requesting "such further and alternative relief as plaintiff may be entitled to".  This is a direct result of the formulary procedure, as relief that was not asked could not be granted because the judge was limited to what the formula instructed him to do ("If you find in favour of the plaintiff, order X, but if you find in favour of the defendant, order Y").  Even today a litigant (certainly in South Africa) is bound to his pleadings in the sense that a court can not grant what is not requested (the principle is referred to as ne petita ultra = you can not grant beyond what has been asked).  So some bright Roman jurist managed to get beyond the confines of his pleadings by adding the general catch-all, with the result that if it appeared the relief sought was misplaced, the judge was vested with a discretion to grant a different remedy.  It was much the same as in arbitration proceedings today.

In the period before the formula procedure was adopted (i.e. prior to approximately 200 BCE), the praetor would assist the litigants in devising new actions.  It must be remembered that during the time the legis actio procedure was used, the question as to whether an individual was entitled to relief in law was strictly dependent upon an action existing for the wrong to be adjudicated in law - or to quote the maxim, ubi remedium, ibi ius (where the law recognises a remedy (= an action), there the law is).  In other words, if a litigant does not have a previously recognised action, he has no recourse as a matter of law.  In order to allow for change, the praetores would develop new actions and so assist the citizens (and later, from 242 BCE, also the peregini (= foreigners who were initially not allowed to use the remedies of Roman law) by the institution of the office of the praetor peregrinus) in order to give them remedies for new causes of action.