When making preserves we do not need to be absolutely precise, a small variation on quantities is not such a big problem, but a scale can be very helpful, especially for recipes that require more ingredients that need to be measured.
You can always know how much of an ingredient you have bought, but when you pick it from your own garden is not always easy to estimate the quantity, so a scale is needed. A kitchen scale is good for small quantities, or you can measure them in small amounts.
If a really large quantity is need to be measured, than you can use your bathroom scale. Put the pan or any other large container on the scale, reset the scale to zero and then add the ingredients that need to be measured. Alternatively you can note the weight of the empty pan or container, add the ingredients and deduct the pan’s or container’s weight from the total.