In serial-verb construction, lái (來) and qù (去) function as purposive markers which occur between the two verb phrases. The VP after the purposive l´ai (來) or qù (去) is the focused VP indicating the cause or purpose.
→ 他們上樓去/來睡覺 (t¯a-men sh`ang l´ou q`u/l´ai shu`ıji`ao/ They went upstairs in order to sleep.)
Lái (來) and qù (去) can also function as deictic markers, indicating the location of the speaker.
Reference: Chinese: a linguistic introduction by Chao Fen Sun