The answer lies in the consistency you prefer in your jams. Pectin is a type of naturally-occurring starch that already exists in raspberries. It's found in the cell walls of fruits and vegetables and gives them structure. When used in jams, it helps give the jam a thicker, more spreadable texture. Some fruits are naturally high in pectin, and make amazing jams without adding any commercial pectin or other thickeners at all. High pectin fruits include pears, apples, plums, quinces, cranberries, and grapes. Other fruits have less naturally-occurring pectin. Low- to medium-pectin fruits include strawberries, raspberries, and blackberries. You can make jam from these fruits. However, the consistency will be different. Side note: I used to think that pectin was used to help preserve jams for longer - to extend their shelf life. However, after much research, I've found that there is actually no hard evidence that this is true.