The act of spell casting opens up magic-users to dark forces and draws
the attention of wicked spirits. In order to defend against such
dangers, many magic-users wear garments imprinted with powerful and
esoteric pictograms. Such wards help protect a magic-user against
psychic pollution or opportunistic attack.

To function correctly, the wards must be imposed spiritually between
the magic-user's soul and the magical threat. There are some extremely
powerful magical periapts that can accomplish this task without the
symbolism of clothing, but they are very rare. More common are robes
which stand directly between the flesh of the sorcerer and the external
threat, particularly flowing clothing such as robes, which provide more
space for esoteric symbols.

Just like physical armor, robes provide a defense rating, from 1 to 6.
This rating functions as a bonus to the save versus magic and does not
provide any bonus to physical armor class. Warded robes also function,
by referee determination, as AC against certain special creatures (such
as those that are ethereal). In Hexagram, the magic defense trait bonus
does not stack with the bonus from warded robes, though the magic
defense trait may be used to protect a companion (see the path of
wonder trait descriptions).

The symbolism of clothing requires that the robes bridge the space
between flesh and threat, so conventional armor may not be worn at the
same time. However, most magic-users fear arcane assault more than
physical assault (which may be guarded against by certain abjuration
spells and loyal warders). Thus, many magic-users opt to wear robes
rather than traditional armor (though they may forego wards and wear
armor if desired). There are legends of warded mage armor, particularly
among the faeries, but such treasures rarely come without strings
attached.

The base cost of warded robes is as follows, by robe level. This
includes the cost of materials and the labor of skilled tailors. Robes
take one week per level to create.

10 gp
100 gp
500 gp
1000 gp
5000 gp
10000 gp

In addition to the saving throw bonus, warded robes may have up to N
special wards (where N is equal to the level of the robes) which can
provide special protection. The nature, cost, and availability of these
additional wards are up to the referee, but often require special
materials or far away knowledge.

Robes must be attuned to a particular magic-user. The cost of attuning
a set of robes to a new owner is half the original cost. Additionally,
the magic-user must cast a spell of level N as part of the attuning
procedure. Warded robes that have not been attuned function at
half-level (rounded down) until the garments are reworked to fit the
soul of the new wearer. Some magic-users may also work curses into
their garments (these would fall under the category of special wards)
as insurance against theft.

In addition to psychic defense, warded robes also have social value.
They advertise to strangers that the character is a magic-user. Such
open magic-users are assumed to follow the laws of the ancient academy
and to reject black magic such as diabolism and necromancy. One of the
responsibilities of magic-users following the ancient rules is to hunt
down and punish sorcerous rebels. Many magic-users are quite zealous in
this task, in order to protect their fragile reputation. Within the
society of magic-users, the complexity of robe symbolism also functions
as a crude method of assessing another magic-user's skill and power.

It is assumed that there are only three kinds of magic-users that do
not wear robes openly. Those that are too poor or inexperienced to
craft proper garments. Those that wish to conceal their nature (this is
considered evil and is a crime in many places). And those, like the
arch-mages of myth, that are powerful enough to need no external
protection.

http://untimately.blogspot.com/2012/10/wizard-robes.html

Cos nobody wants to see Poindexter naked.

Well, according to Harry Dresden, it's really 'cause their labs are always cold.

They just got out of bed and are perpetually on their way to the bathroom to complete Their ablutions.