Why do tablets and e-readers need 3G and USIM?

Tablets and e-readers need 3G and USIM so you can download information over the wireless networks, even if you are traveling abroad, quickly and securely. 3G requires the USIM to provide additional security compared to the SIM. With 3G and the USIM, you are ensured a secure encrypted connection to and from the tablet and/or e-reader when you are transferring data or surfing the Web. The USIM is used to connect to the 3G wireless network, which is essential for quick transfer of data like e-books, videos and music.  

See: What is the difference between SIM and USIM cards?