Debating is mostly about the future. Motions begin 'This house would ...', meaning they look ahead. Debaters either embrace (if the proposition) or resist (if the opposition) the changed future that is offered. Either way, they're looking ahead.
But sometimes it is good to look back. Sometimes what happened in the past can inform the way you see the future. That's when you can tap into the power of precedent.
Precedent can be useful by providing examples from the past of measures that are similar to the one proposed by the motion, and showing how they either worked (if you are the proposition) or didn't work (if you are the opposition).
Let's take a couple of examples, one for the proposition, one for the opposition.
Say the motion is This house would ban the advertising of junk food. The proposition can cite the banning of tobacco advertising in the UK since 2003. It has meant that while smoking is still legal, it is now much less visible and there is much less incentive for people to take it up, leading to reduced numbers of people smoking and consequent health benefits. What has worked for smoking could work for junk food.
Say the motion is This house would make tobacco illegal. The opposition can cite the prohibition of alcohol in the United States from 1920 to 1933. Despite its good intentions, prohibition didn't stop people drinking alcohol; it simply drove alcohol underground. Alcohol was supplied by criminal gangs, who greatly profited from the bootleg trade and grew in power as a result, often engaging in violent conflict with each other for territory. Drinking actually became more dangerous because it could not be regulated. Making tobacco illegal could have the same outcome.
Make sure your precedent is similar enough to the measure proposed to make the comparison valid. So, if you are opposing the motion This house would remove books which use discriminatory language from school libraries and syllabuses, don't say that this is just like the Nazis burning books. It isn't. The Nazis burnt books because they hated the race of the people who had written them. In this motion, both sides are (you can assume) opposed to racism. They just disagree about the best way to deal with racist language: ban it or engage with it? In general (as Gary Lineker has recently found out ...), unless the motion specifically mentions the Third Reich, it's a good idea to avoid mentioning the Nazis in a debate. It is nearly always a disproportionate comparison, and may also be offensive to victims of the Holocaust and their families.
Of course, in order to be able to refer to the past, you first need to know something about it. A successful debater will take a keen interest in history, both formally as a subject in school, and also informally, reading history books and magazines, watching historical documentaries and listening to history podcasts. They will also take a keen interest in current affairs, as what is current affairs today becomes history tomorrow (and the good news is that the longer you live, the more history you will have experienced). As the slogan of the magazine History Today says:
What happened then matters now