Normally you'd remove the post & install it upside down (if possible for
that model), then you have the head to make adjustments with. That screw
might be for attaching a hook to hang weights for stability.

You can screw the pan/tilt/ball head to the bottom of the center
column.  Some 'pods have a reversing center column instead of
a screw at the other end.  Some have 1/4-20 at one end of the
column and 3/8-16 at the other - a feature that is incredibly
un-useful.

You don't say what sort of macro work you will be doing.
For the usual "flowers & stuff" I would recommend a table-top
tripod, a tripod that splays its legs and has a short
center column or one with a tilting center column.

Flowers in pots that you can put on the table are a real
convenience.  Otherwise consider a small gardener's stool
and kneeling pad.

A right-angle finder can get some nice close-to-the-ground
perspective without having to get one's chin in the mud.  For
really down-low shooting you don't need a tripod, just rest
the camera on the ground or on a small sandbag.

Don't forget to visit the florist: great
selection of fresh flowers and they'll sell you one flower
that you pick out if you explain you want it for photography.