зачем

Russian

Univerbation of за чем (za čem), from the preposition за (za) + чем (čem), instrumental singular of что (što, “what”). This was originally marked as Saint Petersburg dialect, while in Moscow зачто́ (zaštó) was used as late as the 18th century.

заче́м • (začém)

(interrogative) why, what for (for what purpose, to what end, with what goal)
Заче́м ты ей звони́шь? У неё телефо́н разряди́лся. ― Začém ty jej zvoníšʹ? U nejó telefón razrjadílsja. ― Why are you calling her? Her phone ran out of charge.
(indicative) why, what for (for what purpose, to what end, with what goal)
Малыша́м тру́дно объясни́ть, заче́м ну́жно чи́стить зу́бы. ― Malyšám trúdno obʺjasnítʹ, začém núžno čístitʹ zúby. ― It's hard to explain to children why they need to brush their teeth.

Заче́м (Začém), while most often translated into English simply as "why" like почему́ (počemú), enquires about the goal/objective of something. It is quite often used in a similar manner to "what's the point of" (како́й смысл (kakój smysl)). Sometimes it is interchangeable with почему́ (počemú), but other times one is more appropriate than the other.
The reply begins with что́бы (štóby) or зате́м, что́бы (zatém, štóby), "(in order) to".