Why is it good to weigh your guinea-pig regularly?

Weighing your guinea-pig regularly is a good thing to do because guinea-pigs hide illness well but they often loose a lot of weight when ill, so weighing your guinea-pig regularly as well as looking out for signs of illness can help you find out if your guinea-pig is ill and treat the illness.
Also, if your guinea-pig is young/a baby you can make sure he/she is developing normally and growing in the expected way. They can be weighed on kitchen scales and will normally just sit on them, although some skittish guinea-pigs may try to run off so you could persuade them to stay on the scales by giving them some vegetables.