Some of the benefits of using IM over other communication (email or phone) options:

• Instant free and fast one to one communication with fellow colleagues in a private window.
• Fast messaging for natural flowing conversation, useful if you want to ask a quick question.
• No cost of a phone call.
• Reduces the email in your inbox.
• Presence indicator makes it easy to see who is available.
• Discrete way of communicating when in meetings.

You don’t have to open Skype for Business to have an IM conversation with people in your organization’s address book. You can start or accept an IM session with someone directly from Outlook on the web.

Microsoft provides extensive documentation on how to use this feature.