For two reasons: A tomato has four different kinds of tissues. The outer thin skin, next to it is the fleshy fruit wall, then the central pith and finally a semi liquid jelly and juice surrounding the seeds. The fleshy fruit wall contains most of the sugars and amino acids but the aroma and flavouring compounds in a tomato are concentrated on its skin. So the flavour of a tomato will vary, on how much skin and flesh you peel or retain. Many chefs, remove the skin and the seed jelly of a tomato when cooking. This makes the tomato less watery, but changes the tomato flavour.
An important compound, that gives tomato its ripe flavour is 'furaneol'. Furaneol also contributes to flavours of ripe pineapples and strawberries. It has a savory-caramely taste. Furaneol, develops in a tomato, as it begins to ripen on the plant, but today most supermarket tomatoes are picked and shipped while still green and artificially stimulated to redden by treating with ethylene gas. Therefore, you end up with a flavourless product. So next time, in a supermarket if you see any thing red with tomato label next to it, don't expect it to be a real tasty tomato. For a real tomato dish, it should smell and taste like a tomato, not just look like one. Duh!!
For long people believed that tomato leaves are poisonous, but today we know that it is a myth. Tomato leaves contain an alkaloid called tomatine. Recent research has indicated that tomatine binds tightly to cholesterol molecules in our digestive system, so that body absorbs neither the alkaloid, nor the bound cholesterol. It thus reduces our net intake of cholesterol- So garnishing your food with tomato leaves is a good idea! A green tomato also contains tomatine and has the same effect like the leaves.