What are the benefits to the overall economy and consumer of horizontal integration? I understand that limited vertical integration can lead to a lowering of prices for products and services, but it seems to me that even limited horizontal integration only leads to less competition.
Horizontal integration may lead to lower costs by profiting from economies of scale. If the market demand for a product is spread over many enterprises, the costs will be generally higher because there will be more fixed costs in total. Instead, if there was a single company, fixed costs would be comparatively smaller.

The effect on the overall economy would be good. Lower costs mean more money for the company and/or lower prices for consumers. The effect on the consumers is not clear.