Farming is perhaps the premier use of water in Minecraft, and using water’s behaviors to make your farming easier is very important.
First of all, you need water to make plants grow. Trees are an exception to this, of course, but anything else you have an interest in growing is going to need you to place water near it. Water will reach very far through tilled soil, up to five or six blocks depending on how you count your distances. Tilled soil within that distance takes a darker color, marking its fertility. This not only allows plants to grow better in that dirt, it also protects the tilling effect from randomly vanishing as it is otherwise wont to do. Keep in mind, though, that water will not affect tilled soil above it in quite the same manner, and does not reach as far. This makes farming with concealed water a slightly more difficult idea to follow through on. Interestingly, the tilled soil and water effect doesn’t seem to care terribly much whether the water in question is source water or a water flow. This can save you a lot of trouble, buckets, and trips back and forth as it means a single water source block supplying more than a 12x12 area with enough water for growing plants.
Second of all, water can be used to gather wheat. This doesn’t seem to make sense at first- but building your farm in the right way can let you use the flow of water to make harvest time as easy as pie. This is because water flowing through wheat will uproot it- just the same as striking it. Curiously, this does not work on sugarcane, nor does it work on melons or pumpkins (though it may uproot the vines, making water harvesting a poor choice for those crops). Using that and the fact that water flows seven blocks, it’s easy to construct a tiered wheat farm where the unblocking of water sources can harvest the wheat you’ve grown without you needing to wander out into the field and strike each block separately.
In fact, it ought to be possible to use this along with the fact that sugarcane blocks water flow to set up a farm where breaking off your new sugarcane growth also harvests your wheat- though I’m not sure at this point how exactly one would do that.
Third, if you’re growing ‘crops’ of wood, you can use water to set up an orchard where the apples and saplings that drop from leaf blocks will fall into water and automatically gather at one end of the orchard for you to easily pick up. Growing trees over water on strips of hovering soil isn’t terribly difficult to manage, and the extra source of food (apples) makes doing this more than worthwhile, even when you could instead craft a tree ‘farm’ that collects only the wood blocks and doesn’t allow leaves to spawn at all.