Experienced son-in-law from Oz here. Wife was a farmer’s daughter from a very dry part of the country. The problem is, water that hits the ground (and doesn’t soak in) will naturally drain away via the most optimal path. Over time, this optimal path tends to become more defined - hence streams, rivers etc. Eventually you wind up with gullies, natural drains and ditches, naturally eroded channels all over the place, which is very efficient at removing water from the land.
Now, while this is a natural process, it is not ideal for farming large fields in a predominately dry environment. So drainage banks are designed to make the water run across the field in multiple small channels - giving the water the maximum time to soak into the ground, without having it all soak into the one small stream bed. If there is excess water from a single large rainfall event - too much to soak into the ground - the banks can eventually direct the water to a holding dam for later use (eg, stock watering).
I learnt all about this over many dinner-table talks, and even drove the tractor with the blade attached to build a few banks. Naturally, I was poor at this and had to receive much instruction on what I had done wrong.