Synthetic sleeping bags are ideal for campers and outside-sleepers not overly concerned with weight. If you're going on a 5 day paddle instead of a backpacking trip, you're going to want to look into this whole synthetic fill thing. A synthetic fill sleeping bag can retain its warmth when it's wet. Synthetic bags also super fast drying, because moisture gets trapped between synthetic fibers, not inside them. Laying a wet synthetic bag out in the sun for an hour or less will usually dry it out pretty well.
That said, synthetic fill sleeping bags tend to weigh more than down bags of the same warmth rating. If you don't need a terribly warm bag, you can get away with a synthetic bag that's still really light. But if you need an alpine bag for winter temperatures, a synthetic bag might be heavy and bulky. Synthetic bags can be less expensive than down, so as you can see, there are definitely pros to choosing a synthetic sleeping bag.
Look out for the temperature rating when you're choosing a synthetic sleeping bag. Temperature ratings tend to be on the lower limit of the EN Test rating, so a 30 degree bag might not keep you toasty in 30 degree weather. Other factors influence how warm you'll be like your sleeping pad, baselayer, gender, hydration level, and whether you just generally tend to be warm or cold when you sleep. So, I always tell people that when they're picking a bag, regardless of its fill material, they should go with a temperature rating that is lower than the temperature they're planning to sleep in. If nights are going to dip to 40 degrees, you could need a 30 or 25 degree bag. You might need an even warmer one. Remember, you can always vent a bag, but you can't add extra insulation.
If you're trying to choose a sleeping bag, check out this sleeping bag buying guide
that I wrote.