Probably the same motivation Yahoo had to release Hadoop, FB to release Hive, Netflix to release so much of their libs and so on and so forth:

- if nothing else, it does no harm (no 'secret sauce' competitors could benefit from)

- it buys karma (think recruiting goodwill)

If the project catches on though then there are many advantages:

- it can spark a self-sustained ecosystem that can further drive the product, at much lower cost for original creator (think Hadoop leading to Cloudera, Hortonworks etc). Product improves, bugs are fixed, toolset matures

- newhires come with know-how to use your internal tools, lower ramp up, better productivity. Anecdotal, but when I was at Microsoft no newhire knew how to use the internal Cosmos stuff, and even among old timers more folk were familiar with Hadoop...