You have 350 kilogrammes of potatoes but you need to know how many grammes that is. To get the answer multiply by 1000 which gives you 350000 grammes.
You've got 270 kilogrammes of rye but someone only wants to buy that in units of grammes, to change the units multiply by 1000. Now you have 270000 grammes.
To convert 680 kilogrammes into grammes multiply by 1000 giving 680000 grammes.