Do I need to add pectin to make jam?

I would probably just follow the recipes. If they're good, the amount of additional pectin they call for is what's necessary to get the jam to set well. If you use less than that, it might end up really runny.

Different kinds of fruit have different amounts of pectin, and it's not evenly distributed through the fruit. For example, apples and pears have a decent amount, while grapes and strawberries have very little. Oranges have some, but orange peel has a huge amount. There's a partial list in the Wikipedia article on pectin.

So, it's possible to make things that get the pectin they need from the fruit, especially if they're cooked enough to thicken and set more easily. Orange marmalade might be the most obvious example of this. But some recipes/fruits don't have enough to ever set, so they need additional pectin.

Since you're searching broadly online for recipes, you may also be seeing variations in how firmly set they come out. You might discover that some of the no-pectin or lower-pectin recipes just don't set as firmly, because that's what the author prefers. If the recipe comes with really good photos, you might be able to anticipate this.

this answer is an addition to Jefromi's answer

To extract and activate the pectin from fruit, you need a certain cooking time - that's one of the reasons our ancestors cooked jam for up to two hours or until it visibly thickened. And the fruit will likely taste different after such a long time, which may or may not be desired. Modern recipes with added pectin or a store-bought pectin/sugar mix will set after as little as two to four minutes at a rolling boil.

So flavour and convenience are two reasons recipes may call for adding pectin even in cases where the fruit would contain pectin for the jam to set.

In addition to the two answers already posted, I thought I should mention that pectin is not present in all fruits in great enough quantities to thicken jam, which is another reason a recipe may call for pectin.

Generally speaking, the sharper fruits tend to contain the higher levels of pectin, such as apples, raspberries, and any citrus fruit. Fruits likely to need extra pectin are more mellow, like ripe pears and strawberries. Some fruits even have changing pectin levels as they ripen/mature.

Blackberries are a fun example - when they first ripen they are full of pectin, but when left on the vine for a few weeks the pectin levels drop significantly. So whilst it's usually a good idea to stick to recipes when making jam, it may sometimes be worth tweaking them a little if the jam isn't setting well enough/setting too well. I've found the best information on fruit pectin levels tends to come from brewing forums, as pectin can be undesirable to some brewers.

Another side note - freshly squeezed lemon juice can be a tasty and effective way to add more pectin without buying the powder!

No, you do not need to add pectin. As you said.... most fruits do have pectin. But you can add lemon juice to help breakdown and release the pectin in the fruit.

I recently made about 6 Jars of Strawberry Jam and added lemon juice without any pectin and it worked just fine.

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You can also purchase Pomona's Universal Pectin that you can use in little to no sugar added jam. It is activated by calcium water which is included in the box (dry powder) . You bring your jam up to boiling then add the prescribed amount of pectin stir to combine remove from heat and jar. I buy mine at Whole Foods or you can purchase online and I believe from Pomona's website. I've never used the other pectins so I can't comment on them but you have to add enormous amounts of sugar and usually the entire box of pectin . With Pomona's it's just the opposite and no long cooking times.