LinkedIn has many features to help a business network and grow. A LinkedIn group is just one of those tools. Your company can create their own LinkedIn groups for several reasons.
They could create one LinkedIn group for employees so the company can communicate company issues with employees. This could be such things as when the company picnic is or why we don't wear sandals to work. Or it could be for more important things like how to use your health benefits wisely.
Another reason to use a LinkedIn group is to open a line of communication between you and your consumer. This could be for such things as talking about a new line of clothing that your company is putting out. Or you can ask for an opinion about which color the bottles your companies juice should come in.
A LinkedIn subgroup allows you to break down each of your LinkedIn groups into smaller bits, or chunks of groups. Easier to manage, these subgroups will also give you the ability to target specific people and ideas.
Now you have the ability to break your big LinkedIn groups down into smaller LinkedIn subgroups. Take the employee example above and break it down. Create a LinkedIn subgroup for employees interested in working on picnics and office parties, one for safety (every company's interested in safety), one for what to wear in the work place, one for heath benefits. Get the idea?
Now take your customer group for example. You could have customers who are interested in helping you design the bottles and boxes your product comes in. Other customers may be interested in asking questions about shipping issues. You could have a whole customer service subgroup if you wanted.