It's handy when applying the same design to certain tags. Rather than creating a load of different ones like:
Code:
h1 { }
h2 { }
h3 { }
You can have it all together - makes the css document smaller.
EDIT:
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, it depends what style they are giving to them tags. They could be applying a margin of 0 and padding of 0, then they may want a padding of 10 applied to something else inside the <body> tag