why we need to tempered chocolate for making garnish?

Answer 1

Hey now...

Tempering Chocolate
When melted chocolate returns to solid form the cocoa butter in the chocolate forms a crystal structure. The strange (or cool depending on who you're talking to) thing about cocoa butter is that the crystal structure they take on depends on the temperature at which they are formed. If the chocolate is allowed to cool on its own, the crystals of fat will be loose, resulting in a chocolate that is dull in appearance, soft & malleable, and greasy to the touch.
This loose crystalline structure has a slightly lower melting point than tempered chocolate crystals. If, instead, while cooling, the chocolate is kept at 88°F (31°C), the loose crystal structure will not form (88°F is above the formation point of the loose crystals).
At this temperature the cocoa butter actually forms a dense crystalline structure. Holding the chocolate at this temperature and stirring will allow a whole bunch of these stable crystal structures to form providing a lot of seed crystals to form in the chocolate. When the chocolate is finally allowed to fully cool, if there are enough stable seed crystals, then the chocolate will harden into a very stable hard chocolate with a slight sheen, snap when broken, and will keep for months at cool room temperature.

Tempered chocolate provides enough stability to be worked into a variety of shapes - sheets, painted onto leaves and peeled off, flowers, cups, and molds. It also helps prevent the cocoa butter from rising to the surface of the chocolate and blooming into unsightly light brown markings or coatings.

With Ganache the tempering of the chocolate prevents
the mixture from 'setting-up' before the blending is complete.
Chocolate that is tempered has a smooth texture, a glossy shine and a pleasant “snap” when bitten or broken. Chocolate that is not tempered might be cloudy, gray, lumpy, and sticky at room temperature.

Smiles

Answer 2

So the cocoa butter doesn't come to the surface and leave a milky looking substance on the chocolate.

Even if the milky substance does appear, there is nothing wrong with the chocolate. People just seem to think it is better without it being there.

: )