If for example you wanted to add to an objects weight (such as a sack) and had a weight counter which increased by a certain number depending on what you put in the sack.
If you had bag A that weighted 3 and wanted to add bag B which weighted 5 then the weight counter would need to increase from 3 to 8

Just found the answer by experimenting

sack.weight = 3
sack.weight = sack weight + 5