> Their motivations in doing this (rather than other things they could do
> with the resources) seem a bit abstract. From what I understand, it is
> not possible to move into the newly created universe.
Actually it may be possible. There is a lot of research that still needs to be done with regards to wormholes and baby universes and M-Theory is certainly shedding insights on this matter. Below are three references addressing this challenge.
 Morris and Thorne. Wormholes in Space-time and Their Use for Interstellar Travel. American Journal of Physics v56, p395 (1988)
M Visser. Wormholes, Baby Universes and Causality. Physical Review D v41, n4, p1116 (1990).
KA Holcomb et al. Formation of a "child" universe in an inflationary cosmological model. Physical Review D v39, n4 15-Feb- 1989
According to Kip Thorne, a wormhole can be made traversible (into a baby universe) if there is sufficient 'exotic' matter, a highly stressed matter, with enormous tensile strengths. The tension or pressure of exotic matter exceeds the energy density. We have no familiarity with such matter today, but it existed under conditions of extraordinary pressure in the early universe.