A value entered in the column just right of the table makes a new column, and one entered in the row just below it makes a new row (or, for a formula, a totals row - see ITable.FormulaBelow). The table only grows over cells that are otherwise blank.
Growing down also lets Tab from the last cell of the body add a row; without it, Tab wraps to the first cell of the body.
Which edges of a table grow when something is entered just beside it.