Why are egg whites or meringue powder used in Royal Icing recipes? Is it for
the hardness or maybe the shine?

Don't be confused by the word icing. Royal icing is a meringue.

Meringue powder is a fine, white powder used to replace fresh egg whites in
royal icing. It contains dried egg whites, sugar, salt, vanillin and gum.
When beaten with water and confectioners sugar it has the same
consistency as icing made with fresh egg whites. You can purchase
Meringue Powder in most cake decorating and party stores (like Michaels or
WalMart).

You can use meringue powder for making meringue or in royal icing.

The use of fresh egg whites in Royal icing is not recommended by the FDA
because of the possibility of salmonella poisioning. The dried egg whites
in Meringue powder were processed by heat, removing that problem. If you
must use fresh egg whites, refrigeration is required.

Here is a good recipe for Royal Icing.

Royal Icing Using Meringue Powder:

4 cups (440 grams) confectioners' (powdered or icing) sugar
3 tablespoons (30 grams) meringue powder (can be found at stores that
carry cake decorating supplies)
1/2 teaspoon almond extract (optional)
1/2 cup - 3/4 cup (120 - 180 ml) warm water

In bowl of electric mixer beat the confectioners sugar and meringue powder
to combine. Add the water and beat at low to medium speed until desired
consistency (about 5-7 minutes).

To keep icing soft and shiny add a few drops of glycerin. Color icing
using gel pastes by dipping the tip of a toothpick into the food coloring
and then mixing it into the icing. The icing needs to be used immediately
or transferred to an airtight container as royal icing hardens when
exposed to air. Cover when not in use. Makes about 3 cups

Consistency of Royal Icing to Cover Cookies

To cover or 'flood' the entire surface of the cookie with icing, first
test the icing's consistency by lifting your spoon and letting the icing
drip back into the bowl. The proper consistency is when the ribbon of
icing that falls back into the bowl remains on the surface for a few
seconds before disappearing. Another way is to take one cookie and place
a small amount of icing in the middle of the cookie. Using a small knife
or spatula, push the icing to the edge of the cookie. If the icing runs
off the edge, thicken the icing by adding a little more confectioners
sugar. Do not add too much sugar at once. The right consistency is when
the icing spreads smoothly and does not stick to the spatula or leave
clumps or streaks on the cookie. Light streaks are alright as they will
disappear as the icing dries. This icing will take several hours or
overnight to dry completely.

To Decorate Iced Cookies with Raised Lines -

Once the surface layer of icing has hardened, pipe on lines, dots, etc.
using a piping bag with a #2 tip. If desired, sprinkle colored sugars on
top of the wet icing for a different effect.

Note: If you do not want the lines or dots, etc. to be raised then pipe
the lines or dots onto the surface of the iced cookies before it
completely dries so it will partially blend into the first layer of
icing.