To Be Useful
To Be Of Use, To Be Helpful, To Be Of Service
When you stand up to do your duty you're standing up and finally being useful. That's why this verb means to be useful.
The reading for 役 is the one you learned with the kanji. The 立つ reading is that of the 立つ vocabulary, which you should be familiar with by now.
Masking tape is useful for various things.
Yamashita-sensei's advice is helpful for life.
I'm sorry that I couldn't be much help.