What is actually landing on your shop floor
A car garage mostly deals with rain and snow dripping off a vehicle. A farm shop deals with more than that. Tractors and equipment come in caked with mud after a wet field. Boots track manure across the floor along with it. That is a wetter, dirtier floor than the one under a car, before anything else about the building even enters into it.
The honest starting point is not a rule about farm shops in general. It is what actually happens on yours: what gets washed, what gets rinsed, and how often something other than a dry tractor tire crosses that floor.
Chemicals and livestock make it a different question than a garage
Two things push a farm shop toward needing a drain that a plain car garage never deals with. One is a sprayer or a chemical tank rinsed out between loads, where the water carries something you do not want sitting on the slab or running out under the door untreated. The other is livestock brought inside during a bad stretch of weather, a sick calf or a lambing ewe kept where it is warm and out of the mud, which comes with its own mess to deal with afterward.
Neither of those has to apply to your shop for the building to be worth having, but if either one does, it changes the answer from optional to worth having. A shop that only ever sees a dry tractor rolling in on a clear day is a genuinely different building than one where a sprayer gets rinsed out every week during spraying season.
When a drain earns its cost
If your shop is a genuine wash bay, a chemical rinse station, or a place animals come inside regularly, a drain is worth having. Water and whatever it is carrying need a place to leave the building instead of pooling in the low corner or running back out the door over the mud and manure that tracked in with it. Our shop floors with drains and anchors page walks through the layouts themselves, a trench at the wet end or a single drain near the middle, in more detail than belongs on this page.
The drain and its line have to go in before the stone and the pour, so this has to be decided early, at the same point you are working out where equipment parks and which door takes the daily traffic.
When you can leave it out
A shop that stores dry equipment, gets swept rather than hosed, and never sees an animal inside does not need a drain to function well. In that case we pitch the floor gently back toward the overhead door, so whatever rain or snowmelt comes in on a tire runs back out the way it came instead of sitting on the slab.
That is a real answer, not a compromise. Plenty of farm shops run for decades on a floating slab with a simple pitch and no plumbing under it at all.
This gets settled before we form, not after
Whichever way your shop is leaning, tell us before the pour. A drain has to be plumbed and the floor pitched to it while the forms are still open, and adding one to a finished floor means saw cutting, trenching back into the subgrade, and patching a slab that never quite looks like the rest again.
Describe how you actually use the building, washing, treating animals, or just dry storage, and we will tell you straight whether a drain makes sense for your shop or whether a simple pitch out the door is the better spend.