Yammer is a team communication solution Microsoft already bought, but is way worse at synchronous communication (chat) than Slack, but also better at at asynchronous communication (threaded conversation, like on Facebook).
Microsoft would want to buy Slack for the same reason they spent $$$ on best-in-class products like Yammer, Sunrise, rebranding Acompli as Outlook. Slack's revenue is incredible for a startup, but a drop in the bucket for MSFT (for now). More immediately, Slack represents an entry point for Microsoft to upsell Slack's customers to other Office365 services and Azure services.
Microsoft would want to buy Slack for the same reason they spent $$$ on best-in-class products like Yammer, Sunrise, rebranding Acompli as Outlook. Slack's revenue is incredible for a startup, but a drop in the bucket for MSFT (for now). More immediately, Slack represents an entry point for Microsoft to upsell Slack's customers to other Office365 services and Azure services.