Ensuring that the table width on your page is a percentage will ensure the table sits nicely within the content area. If the table is wider than the content area, the user may have to scroll left and right to view the full width of the table.
Making the table a maximum of 100% will make sure that the table will always fit inside the content area of the site.
The image below shows a table which is wider than the content area, meaning that the user would have to scroll left and right to view all the information held inside the table.
The image below shows the table which has the width set to be 98% of the content area, meaning it will fit inside the content area so the user does not have to scroll left or right to view the content held within the table.