Toyota is far and away the global leader in sales of hybrid vehicles and thus enjoys a cost advantage thanks to economies of scale. By offering components based on the same technology, to other automakers, both Toyota and its competitors can gain a further advantage.
Denso also wants to move beyond these components into the increasingly crowded battery supply market. Denso has reportedly been researching lithium ion battery technology for 15 years, originally with an eye on the consumer electronics market. Now it wants to move that work into the automotive space. That would put it in direct competition with Toyota's own joint venture with Panasonic that makes currently makes all of the batteries for the automakers hybrids.
[Source: Automotive News (subs req'd)]