Tunneling IP traffic over the X.25 packet layer protocol was common practice in the early days of the Internet. Then Deutsche Telekom announced to support AO/DI Internet access. AO/DI (Always On, Dynamic Isdn) applies the multi-link PPP protocol where a permanently connected low bandwidth link uses the isdn D-channel. The D-channel PPP traffic is carried inside an "X.31 Case B" connection, which uses the X.25 network layer protocol and the LAPD data-link protocol.
This announcement immediately created a demand for a cheep method of tunneling PPP inside an X.25 connection for the Linux operating system. (It was possible to do that with Linux before, but only by means of more expensive intelligent WAN controllers which implement X.25 in firmware). Unfortunately, Deusche Telekom has recently (rather silently) withdrawn their AO/DI plans. But by that time the related Linux code was already sufficiently advanced. Further, the code was not tightly coupled to AO/DI and designed to be useful with several other (non-AO/DI) scenarios. Thus, it was worth finishing it.