If the water contains an appreciable amount of dissolved substances (as certainly would be the case when measuring TSS in seawater), these will add to the weight of the filter as it is dried.
Failure to add this step is a fairly common mistake made by inexperienced laboratory technicians working with sea water samples, and will completely invalidate the results as the weight of salts left on the filter during drying can easily exceed that of the suspended particulate matter.
Although turbidity purports to measure approximately the same water quality property as TSS, the latter is preferred when available as it provides an actual weight of the particulate material present in the sample.
Because turbidity readings are somewhat dependent on particle size, shape, and color, this approach requires calculating a correlation equation for each location.
This is because particles above a certain size (essentially anything larger than silt) are not measured by a bench turbidity meter (they settle out before the reading is taken), but contribute substantially to the TSS value.