I think what it will do best is solidify alignment.
I am sure Slack has many goals when it invests, but the companies I’ve been involved with it has invested in are partners that have helped establish portions of their ecosystem.
They’d do it anyway. If it’s in your interest to build on Slack, you’ll do it. Slack has the scale and dominant market position.
But that extra support from a small investment, together with a better inside line to the company, is cherished, at least for a while.
By giving top partners even a small helping hand from a key partner and highly respected brand … those partners will work extra hard for Slack.
That’s a terrific investment.
The risk, of course, is you accidentally favor one vendor over another, and create quiet disalignment with the ones you don’t invest in and promote.