As far as I understand, a DCDC converter is less efficient and more expensive than an equivalent ACAC converter. I don’t know about switching power supplies, and whether that’s true or extendable to the transformer case, sorry.
Long distance point to point power transmission (like internationally) is often DC because transmission losses become more important.






You read the obvious truth. If you’re just getting by, can’t really save anything each month, and then your phone breaks and you need a new one (as in, it’s your only means of communication, so you can’t work and earn money without it) then spreading out costs over several months may help.