pppd is used in various scenarios to create a network connection according to the PPP standard between two systems over serial connections. Several configuration files can be put in /etc/ppp/peers, passwords in /etc/ppp/chap-secrets and /etc/ppp/pap-secrets, and "inet ppp" definitions in the file /etc/network/interfaces.
See manually using GPRS.
Another feature that requires PPP is accessing an OpenMoko device over Bluetooth within the DUN ("dial-up networking") profile.
The GSM modem of an OpenMoko device can be made available for a computer connected via USB or Bluetooth, using a serial device, SSH, or a TCP port. In this case, pppd run on the computer can establish a GPRS connection.
If instead of a real OpenMoko device QEMU is used to emulate one, it can be networked with the host computer via an emulated serial port.
Secure VPN (virtual private network) connections can be established using pppd over SSH or PPTP.