wells in high treason

What's the benefit of building a well?

The wheat is growing very slow.
Once you have many villagers in a village, they have to wait for the growing wheat.
If you build a well, the idle villagers, start irrigate the wheat, so the wheat is growing much faster.
The result is, you have less idle villagers, waiting for growing wheat.

Note:
Sometimes it's better to keep the farming zone small, to better protect your villagers from attacking enemies.
But the smaller your farming zone is, the more idle villagers you have. So a well is needed.