Modems are attacked to change DNS-servers for advertising or infecting systems. This may be done through brute-forcing admin passwords on these devices.
Printers contain passwords which may be stolen through attacks on these systems and used for attacking other network devices (e.g. servers).
The talk by Marco Preuss, Director, Europe, Global Research & Analysis Team, Kaspersky, from PasswordsCon Bergen 2013 focuses on examples of these attacks. Some common attacks may also be utilized for password-testing - dictionary generating.