When is it necessary to use 来 to convey the meaning "in order to"
Simple rule: If you've got another verb after 他， translate 来 as to.

Of course, things are a bit more complex! 


把杯子拿到这里来。(来 corresponds with hither)
把杯子拿到那里去。(来 corresponds with hence)
他用水来浇花。(来 corresponds with to)
他用铅笔来画画 (来 corresponds with to)
他明天来看你。(来 corresponds with ? He tomorrow to see you ?) 
他马上来和我们一起吃饭。(来 corresponds with ? come? to?) 


Nothing ever 'to' but it also 'come', just depends where you're standing!

You can't really use English to make sense of Chinese! Chinese makes sense!