Eh, web traffic can be measured too. App installs aren't all that interesting if people install the app and never use it. Therefore, valuation metrics should be driven by usage (regardless of platform).
- Sunk cost - I would guess people feel a lot more invested in a service after installing an app (and then being forced to make an account, in many cases)
- Mind share - Their logo will appear in your app drawer and they will be able to send you push notifications
It must be effective, because many companies push heavily for app installs. Check out mobile reddit, for instance. Although yes, I think that a lot of this is useless metric driven strategy.
But that doesn't really apply in Slack's case.
I'm not saying you're wrong that that's why they're doing it, just that they should stop. :)
And the second most important reason: push notifications.