A question non-farming people often ask is what is the difference between winter and spring cereals. The crops are the same - wheat, barley or oilseed rape - so why plant them at different times? The reasons include:
Such is the acreage now under the plough, it would simply be impossible even with the bigger, computer-driven machinery to cover the area in the country needing to be sown in one season. The same scenario applies at harvest with different crops ripening at different times - oilseed rape first in August, then the winter barley and spring barley and lastly wheat. The modern farmer can cope with this spread.
The crop may be described the same (wheat is wheat?) but there are different varieties. Some have been developed to be established and grown in low temperatures by being sown in the autumn. The plant is ready to grow rapidly in the Spring which enhances the yield. Winter crops usually produce some more grain than spring crops. Spring crops in contrast do not have to be as hardy, cost less (unless a specific seed variety) and maybe yield less - but much depends on the weather - rain and sunshine needed at the appropriate time. Peas and other crops have not yet been developed to survive winter conditions, so their place in the farming year is in the spring.
Converse to the higher yield, winter crops have higher costs. There is more tillage work and more spraying, in particular due to the longer growing period. Costs are also incurred earlier in the year so that the farmer has a greater cost to carry forward until the harvest return is received.
Farmers vary their cropping schedules so that their fields have a rotation of the plants they grow. This helps fertility as good management of their fields keeps the land in good condition. Different soils across one farm also encourage a variety of crops. In addition, sometimes there is time to put in a 'cash crop' - eg stubble turnips after winter barley which can be folded (ie fed off) by sheep or cattle, adding humus and organic manure to the ground and helping the preparation of the seed bed for a productive spring crop.
You rarely get a consistent weather period for weeks on end, so interruptions to sowing and harvest often occur. If all the cereals were ripe at the very same time and could not be harvested for all the reasons given except in a six-week period, then the crop would deteriorate and even perish. Different varieties and different time zones in winter and spring spread the crop life and allow a successful six-week harvest period. Even if in a dry year it's only five weeks or in an inclement year, it might extend to seven - the crop is still safe.
Winter stubbles are enjoyed by wildlife - birds and partridge in particular. They offer a food supply from the fallen grain and to a certain extent some natural cover.
So - a number of reasons - there never tends to be just one! And perhaps choice comes in to it as well. It can be simply what suits the farmer for his own land and own satisfaction. A further example of the many aspects of farming life.