It's not 100% accurate, but most people treat it as equal.
You do need to be careful with foods such as icecream. Doesn't work quite the same for that. For example, 500ml = about 400g of icecream if I remember rightly.
It is, but the denser the liquid, the more inaccurate it becomes. Most everday fluids are fine on a 100ml=100g basis.
Makes it easy if your weighing foods as part of your diet because you can just use your normal weighing scales if you have to add milk, water or similar Great if you are doing a all in one recipe. Bowl on scales, just keep adding stuff