In some situations it can be useful to have a cordless phone in the office allowing the user to roam around the office while they're on the phone.
Because analog cordless phones are fairly inexpensive, if you combine a cordless phone with an ATA, you'll can combine the PBX features with the mobility of a cordless phone for relatively cheap. This is compared to buying a proprietary cordless phone that only works with a particular PBX vendor's equipment.