So, it is different for everyone, but as a general rule when you first start, you do need to eat at least some, if not all your syns to loose weight. Its to do with metabolism i think. I find i loose more if i eat about 80 - 90 syns a week. I tried limiting to less than 50 one week and maintained - i also loose if i have all 105 syns.
Also, at some point you stop losing weight, and generally people find at that point they can kickstart losses by reducing syns. If you aren't eating very many already, you will start to reduce free & superfree foods - and then you will be hungry - and then you will be more likely to fall off the wagon.
Finally - 1 syn is equal to 20 calories. You need to eat 3200 extra calories in a week to gain a pound, so eating them all, at least in the first instance is not going to result in a gain.
Syns are an integral part of the SW plan!