For one, you can never guarantee nor should you expect your team to be colocated. Even if you have one office, people should be free to work from home. Side chats get lost, people forget or don't hear. Slack is public and searchable. No mumbling and if you mistype, you can fix it. Plus you can add documents, links, images while you're talking.
People say getting communication on their phones is like an anchor to their desk but I feel the exact opposite. It means I can leave my desk anytime I want and not be out of communication. If something critical needs my attention, I can answer while I'm home and not go back to work.