To help stop the aircraft in the event of brake failure, or some similar
accident leading to a runway overrun.  Just past the end of many military
runways, you'll find an arrester cable strung across the field.  The cable
(unlike those on aircraft carriers) isn't attached to anything firm;
instead, each end is linked to a long chain, which just drags on the
ground.  The idea is to slow the aircraft down in a reasonable distance;
the tailhooks on Air Force fighters are smaller and weaker than the
superficially similar hooks on Navy planes.

The inevitable next question, "Does this mean Air Force planes could land
on a carrier in an emergency?", has been discussed at length in this
newsgroup.  It has been conclusively established that, no, an Air Force
fighter could never land on a carrier because, first, its landing gear is
likely to break in the much heavier touchdown required for carrier landings
(sink-rate figures quoted in the newsgroup give an F-15's main gear roughly
a fifty-fifty chance of taking a carrier landing without breaking); second,
even if it could get on the deck in one piece, the weaker AF tailhook would
break when it caught the Navy arrester cable; and third, even if the
aircraft was physically capable of it, Air Force pilots aren't trained in
the highly specialised and difficult art of carrier landings.

It has been pointed out that, if the USAF thought there was even the
slightest chance of ever being able to save one of its planes by landing it
on a carrier, it would have been tested on the mock carrier deck at
Patuxent River; the fact that this has never been tried is pretty solid
evidence that the Air Force engineers (who would presumably know) are
certain it can't be done.

The F-16Ns used by the US Navy as adversaries in training have the standard
Air Force tailhooks and undercarriage, and are definitely not carrier
capable.

The RAF pilots who learned to operate from carriers in a few weeks on the
way to the Falklands are a different matter entirely; they were flying
Harriers, and of course most of the above is irrelevant to VTOL aircraft.
Some training was still required, of course, but the requirements are very
different, both for the aircraft and the pilots.  (As one Harrier pilot put
it:  "It's much easier to stop and then land, than to land and then try to
stop.")

A few land-based aircraft have been flown from carriers with minimal
modification, notably the Lockheed C-130 Hercules and U-2.  Both of these
were fairly special cases involving aircraft designed for very low speeds
(and, in the case of the Hercules, rough landings) from the start.

On 30 October 1963, a USMC KC-130F made several carrier landings and
take-offs on the flight deck of USS _Forrestal_, in a series of tests
intended to determine whether it would make a good COD (carrier on-board
delivery) aircraft.  The only modification was an anti-skid braking system.
The aircraft made several landings and take-offs, with no use of arrester
gear or catapults, and performed well (the pilot, Lieutenant James H
Flatley III, was awarded the DFC for his part in the tests).  However, it
turned out that the Hercules would have been unable to fit in a carrier's
hangar deck, so the smaller Grumman C-2 Greyhound was developed instead.

Modifications to the U-2 involved the addition of an arrester hook and a
strengthened landing gear (the U-2 already had folding wings).  In 1964 two
modified U-2As, designated U-2G, were flown from USS _Ranger_; the tests
were successful, and several modified aircraft were apparently flown from
carriers by the CIA during the 1960s (the service version may have been
designated U-2J).  In 1969, a similarly modified U-2R was flown from USS
_America_, but this does not seem to have led to any service use.

Land-based aircraft have been successfully modified to be carrier-based;
the modifications involved, when the aircraft is a fast jet, are extensive.
It isn't just a matter of adding a tailhook and new landing gear; most of
the airframe needs to be redesigned.  The best known example in the West is
the BAe/MD T-45 Goshawk, the US Navy's new trainer, based on BAe's Hawk.
The Russians have had some success in adapting several fighters and attack
aircraft for carrier service.  Carrier tests were made by modified MiG-29
and Su-27 fighters, and by trainer versions of the Su-25; the naval MiG-29K
was cancelled, but the Su-33 (based on the Su-27K) and Su-25UTG have
entered service.  A report of an early MiG-29K being torn in half on its
first attempt at a tailhook arrest gives a hint of the difficulties
involved.