The idea is that rational infrastructure projects create additional economic growth in future (i.e. more stuff gets moved across the built bridge, more real value gets created), so the printed money are used to support this growth. So effectively government invests into a long-term project by "borrowing" money from those who are saving them today (in a sense they borrow from "future"). If investments are good, the "loan" gets payed by more taxes being collected. If investments are bad (e.g. as many infrastructure projects in China), they will cause various problems in future since economy gets injected by money not backed by productive activity.