Rolling hay fields soft conditions will smooth out clumps allowing higher field speeds. Rollers punch small rocks into the sod, letting hay cutter bars to get closer to the base of plants. This maximizes yields and reduces cutter bar damage. Hay growers also see less damage to cutter bars, crimping rolls and baler knives.
Rolling corn trash before planting breaks down root balls and firms the soil around trash. Stalks do less planter plugging and breakdown quicker with good soil contact.
Packing with the roller after seeding firms the seedbed for uniform emergence. Any surface trash is flattened so combines don't pick up dirt at harvest and downgrade the sample.
On longer term zero tillage fields, rollers can eliminate the majority of bumps in the field for easier farming.
Smooth rolled fields simplify spraying operation. The smoother ride gives more uniform chemical application, as well as enabling higher field speeds.