To elaborate on what others have said, greasing a seatpost is common for metal frame/seatpost combinations (not carbon). The combination of aluminum and steel will suffer galvanic corrosion if not isolated with grease or anti-seize compound. Same-type combinations of metals are less of a concern, but regular corrosion can still lead to sticking. Of course, for folders it's probably not necessary in a lot of cases, since the seat post is often moved frequently as part of folding.