It may pay to read the fine print, or in this instance just the requirements per the airlines web site.
A thin laptop counts as “one small item” which must be included in your weight limit!
Being lighter than many I might think an extra kilo or two is ok for some. Interestingly you can purchase an extra 3kg of carry on for a little less than the cost of adding 20kg of checked luggage to a starter fare!
Why it is less effort for Jetstar to handle your 20kg of checked luggage than for you to handle the 3kg personally may have already been explained?
Even in Jetstar’s slightly more premium ‘business class’ where it previously was 20kg carry on it’s now 14kg with no single item over 7kg. I guess that still 6.9kg in a large sized cloth shopping bag for the light traveller? More discerning passengers may make other choices.