by my side,英文疑问

by my side,英文疑问
by my side应该是副词性的短语,sit by my side,she is always by my side.
但我也听过不少歌曲或文章里写着I need u by my side,这里的by my side怎么解释?
为什么不写成I nd U 2 be my side??
THX IN ADVANCE