VoIP phones have gone far from its PSTN ancestors. One major innovation is the use of multiple lines on a single device.

In PSTN, one 'line' means a pair of copper wires that can be occupied by a single call. While today, in VOIP, it is not so. A 'line' is a username(1 line=1 username or persona) and most of the phones in the market allows 6 to 8 simultaneous calls per username. 

Moreover, these calls can be limited as desired from less than 8 to a minimum of 1 simultaneous call. 

(For an instance that a 'line' is enabled on a phone, it could receive up to 8 simultaneous calls before the phone will report 'busy'. And if during a call a new call comes in, you will see the new call in the interface and can choose to either ignore or reject the call or put on hold the current call to answer the incoming.)

There are scenarios on how multiple lines are used and you could say that these entail the reason why they were designed so. One for example is to use Line 1 to be your personal line(a direct extension perhaps) and Line 2 to be a general user where inbound calls from PSTN are going. Another is if you want to have different Caller ID's appearing on recipient's end when using different lines. Depending on the desired setup, VoIP phones features can be optimized.