I'm looking at the last example at http://orgmode.org/worg/org-tutorials/tables.html and I'm wondering why that first column is necessary.

the / tells the table exporter that that line is going to be for doing column grouping

the # is just a nil comment character

you can make that exact same table with:

|  N | N^2 | N^3 | N^4 | sqrt(n) | sqrt[4](N) |
|----+-----+-----+-----+---------+------------|
| / |   < |     |   > |       < |          > |
|  1 |   1 |   1 |   1 |       1 |          1 |
|  2 |   4 |   8 |  16 |  1.4142 |     1.1892 |
|  3 |   9 |  27 |  81 |  1.7321 |     1.3161 |
+----+-----+-----+-----+---------+------------|

I'm not sure why you'd prefer to have the version with the leading column being unnecessary like that....

@tanzoniteblack Thanks for the explanation.