What statistics does the keyword “all” compute ?
Well, any statistics that you ask for.
For example, I have a table of min, mean, and max age of car buyers, grouped by country, size, and sex.

I would like to have the 3 statistics aggregated by the grouping column “sex”,
i.e., I want to have the min, mean, and max age for all the car buyers, irrespective of sex, in the same table.
To get that, drag and drop the keyword “All” onto the grouping column “sex”.
You are in effect, asking tabulate to make an extra group, “all”, for the grouping column “sex”.

If you don't like the word "All" for your report, you can change it.
Double click in the box for "All" in the table, and enter the word of your choice.
In this example, I have changed the word "All" to "Total".

Dropping the keyword “All” on a grouping column tells tabulate to compute the aggregate statistics for that grouping column.
You can do the same for all the other grouping columns in the table.
There is a checkbox in the Control Panel, “Add Aggregate Statistics”.
Checking that will give you the aggregate statistics for all the grouping columns in the table.