Why do cats (and dogs for that matter) have an extra footpad on their front legs?

I assume you are looking at that funny little footpad that sits up
higher than the rest of your cat's toes.

This is what is left of a residual "heel" (more like the heel of your
hand than your foot.)that has evolved up higher in cats and other
4-legged creatures.

Press you fingers flat against the floor, palm lifted toward you.
Think of your cat as walking this way. Up where the heel of your hand
is, is the same as where your cat's extra pad is.

Check out this site to see how a cat or dog's leg is formed -
http://www.specificbreeds.com/html/skeleton.html

Bad pics, but good description here
http://www.oricomtech.com/projects/leg-anat.htm#Cogs2

Hope this helped!

-K~

It is called a "dewclaw".

http://www.dictionary.com/search?q=dewclaw

It is vestigial. Evolutionarily speaking at one time it was probably a
useful 5th toe but now it is just a left over or a "vestige". It has
no real purpose any longer.

Dogs and cats are digitigrade animals: they run on their digits,
"tiptoe". This increases their stride length, and thus their speed. A
page on legs, feet, and running from the University of Michigan
Department of Zoology:
[ http://animaldiversity.ummz.umich.edu/anat/running_fast.html ]
And a comparative look at digitigrade legs:
[ http://animaldiversity.ummz.umich.edu/media/anat/feet_and_legs/posture.jpg
]

The extra footpad is the "carpal pad." It is associated with the
dewclaw, a vestigial fifth toe evolving from the thumb and big toe.
(Properly, though, the dewclaw refers only to the claw on the hind
legs). The carpal pad is primarily an evolutionary remnant (like
hipbones in whales). However, some researchers argue that the carpal
pad is useful for jumping, high-speed running and quick changes in
direction, when the leg is extended and the entire foot touches the
ground. (Note: the carpal pad is *not* anatomically related to the
heel, which is higher up on the leg).

A short summary of PhD work by Bonnie Dalzell, a functional anatomist
and paleontologist, on dewclaws and their purpose in dogs is found
here:
[ http://www.batw.com/netpet/articles/dewclaws.html ]
"In my PhD work I found that really fast, slender legged dogs
hyperextend the forefeet down to the stop pad when at a full gallop.
They also roll slightly medially onto the inner surface of the carpus
(pastern) when running. Examination of tracks in firm moist soil and
examination of the feet of dogs running in grass show that the first
digit of the forefoot (the thumb) comes into contact with the ground
during high speed locomotion. In fast galloping dogs with small tight
thumbs the claws wear with racing so that they do not need periodic
trimming."

Jared M. Diamond mentions the role of the carpal pad in cats while
jumping in Nature, vol. 332, 1988, p. 586-7:
"Cats have a single carpal pad that lies behind and above the other
pads on the feet, and does not come into contact with the ground. This
pad does not play a part in normal locomotion, but is thought to have
a role when the cat lands after a jump."
[ http://www.science.edu.sg/ssc/detailed.jsp?artid=672&type=6&root=4&parent=4&cat=35
]

I'm pretty sure that my cats use it while climbing as well to brace themselves...