Answer 1

It is used when you want a table cell <td> to span over its regular size, to another column or row.
Example:
+------+------+------+------+
| | | | |
+------+------+------+------+
| | |
+----------------------+------+
<table>
<tr>
<td></td>
<td></td>
<td></td>
<td></td>
</tr>
<tr>
<td colspan="3"></td>
<td></td>
</tr>
</table>