ATIS is the airborne dissemination of the *observed* equivalent to the automated METAR or SPECMETAR weather report with the addition of NOTAM L's and NOTAM D's.
The FAR's require a pilot to know the wx at all airports of intended use and giving the code to the approach or ground controller on initial contact assures him you are briefed on the weather and NOTAMS at the field. if you report unable to copy the ATIS the controller must read it to you. Listen to Newark ground control, they are VERY anal about this. they will not let you taxi past Bravo short Kilo without getting the latest numbers which has undoubtedly changed since pushing from the gate.
Also, while the ATIS is still broadcast on VHF voice unless your in a cessna at a podunk airport, no one tunes that in any more...90% of the air carrier traffic at major airports gets the ATIS via ACARS data link. a typical ACARS ATIS message as seen in the cockpit for Newark in March looks like this:
KEWR ATIS INFO M
231545Z
30021G34 PSM6 SCT012 BKN040
06/M02 A3021 (THREE ZERO TWO ONE)
APP IN USE ILS 22L CIRL RW29. LNDG RW22L RW29.
DEPG RW22R/W. AVAIL DIST RW22R/W 10,500.
TXWY B CLSD BTW TXWY RH AND RJ. UNLIT TOWER 2NM SOUTH. BIRDS VICTY
CONTACT FLOW 134.25 5 PRIOR TO PUSH. CONTACT GRND CNTRL 121.8 FOR TAXI
ADVISE ATIS M