That is a swather.
Hay goes through conditioning rollers to crimp the stems so the hay dries faster. For fine stemed grasses crimping might not help much. Tremendous difference on alfalfa & sudan type crops.
Don't have to be looking back all the time.
Most tractor mounted rigs have a much shorter bar, so takes a lot more running ti cover the same acres.
If comparing to a bar mower, the reel keeps the bar from plugging.
No running over hay when opening fields, so better for small od shaped fields as well.
Some are going to the rotating cutters. They drive them like they stole them, cover a lot of ground, use lots of fuel, takes lots of horse power, not likely to plug, leave a ragged cut, especially on shorter crops, regrowth can be slower.
Cost more. Need smother feilds than I have to drive that fast!
Here we cut with a 18 ft head on swather with a sickel bar, if hay is heavy, may only cut 14 ft to 16 ft.
When top dries, rake 2 swaths togather side by side, let dry some more. bale with a wide pickup head baler .
Very few big squares around here. What is baled that way is probbly for dairy, export, or trucking long distance.
They need to be stored inside, or tarped.
Hardly any big round bales around here ever are stored under cover.
If we need rain, go cut a lot more hay, down , if we are lucky, we will get more than .01 of rain before it all dries .
There are also pull type swathers.
Some guys put a swather head on a bi directional tractor, and pull a pull type hydroswing swather behind , usually both are 16 ft heads.