It is unclear what Alphabet plans to gain from this investment, although it seems that it might have to do with self-driving cars and technology. Interestingly enough earlier this year Waymo and Lyft announced a partnership to develop the technology, and a couple of months later, Lyft announced that they would be looking to develop their own technology as well.
Presumably this investment could potentially give Alphabet a share in the company and if and when Lyft begins licensing out its self-driving tech to other carmakers in the future, Alphabet would stand to gain as well. However Bloomberg does note that this investment might not even happen since nothing is confirmed yet, so for all we know it may never come to fruition.
Also Bloomberg points out that Alphabet was an early investor in Uber, which is going to be tricky since not only is Uber a competitor to Lyft, but Waymo and Uber are currently embroiled in a lawsuit over alleged stolen intellectual property.