It could be with a view to learning what they are up to (e.g. new products or product updates, their thoughts on industry direction, or any other reason). I said organization rather than company, because it does not have to be a for-profit company, could be a non-profit.
Some other reasons could be: to know if they are hiring in an area you have skills in (although that could also be found from their jobs page), and because they are competitors.
