An ATA gateway allows us to connect any analog phone device to our 3CX Phone System. In essence, it converts from an analog phone signal to SIP, letting us use, for example, any standard analog phone handset with our 3CX IP PBX! You could use your long range portable phone, a bunch of handsets that seem a waste to throw away, a fax machine, or any other analog phone device. Once again this type of gateway allows us to use equipment that we've already invested in.
Yes, you can connect an analog fax machine to your phone system but the ATA you select must support T.38 protocol. Also, your FXO must support T.38 and even then it is tricky to get it all working. I'd recommend keeping the fax machine outside your PBX (if possible) or using the 3CX fax server.
With an ATA gateway no phone is too old to connect to your 3CX IP PBX. An ATA will allow you to convert any phone to a SIP phone.