why should i write the attribute value inside a tag?

Answer 1

if it's not written inside the tag it no longer the property of that tag, may become into content.

Answer 2

It is used to supply the given value to the 'name' attribute. which store the value until 'Submit' .and after 'Submit' pass the value to the server using PHP Like services.

If you are using the attribute 'value' inside a <input> tag .
_*Example:8_
**
<input type="radio" name="buy" Value="pasta"/> PASTA <br/>
<input type="radio" name="buy" Value="cheese"/> CHEESE <br/>
<input type="radio" name="buy" Value="chips"/> CHIPS
<input type="submit" value="Buy">
**
here , 'value' attribute is supply the value pasta, cheese and chips to the variable storage 'name'= "buy" .as the selection made on the radio button on behalf of text PASTA,CHEESE and CHIPS respectively.