The reason for the different styles is
cost and torque.
The slotted head screws are cheap and
easy to make. But they're completely
useless for powered screwdrivers and
you can't put much torque on the screw
without it either slipping out or
stripping the head (and maring the
surface of whatever you're screwing).
[Mad]
Phillips screws are self-centering,
making powered screwdrivers possible.
They're somewhat more expensive to
produce than slotted-head. They tend
to 'cam-out' easily under torque,
making it hard to apply much torque.
I've heard they were designed that way
to prevent overtightning. However,
it's not good for exposed fasteners to
look stripped.
Robertson-head and allen-head
fasteners can handle more torque than
phillips-head fasteners, but are more
expensive. Because the bottom of the
hole is flat (unlike the pointed end
of the phillips), there's more contact
area and so it's less likely to
cam-out. The robertson-head is cheaper
than the allen-head, but the
allen-head has six points of contact
rather than 4, making it less prone to
rounding out the hole.
The Torx-head fasteners solve the
problem of rounding/stripping by
having the flat bottom of the
robertson/allen that reduces cam-out,
but it has much better contact with
the driving bit to prevent stripping
the head. The points of the 'star' on
the driving bit engage the recesses on
the screw at nearly right angles, so
it has a very positive contact. Torx
is becoming more and more popular
because of that, particularly in
assembly-line work.
Because they're less likely than a
phillips to be damaged when
tightening, the allen (internal hex)
heads are often used for exposed
('decorative') fasteners on 'some
assembly required' furniture. It's
also very cheap to make the allen
keys, so they usually include one with
the fasteners.
Best Answer
It's for hanging on a peg-board hook, both in the store and in the shop.