They're not used for formal communication. Emoji are used more for informal / familiar conversation.

Adding a cake emoji to "Happy Birthday" in Slack is nicer than plain text. Younger-staffed start-ups will use emoji alongside "oops", frustration, and/or celebratory statements.

Marketing trying to be "bro"?

The Japanese are more likely to include emoji in their writing, as they are the inventors of the communication style. You might send a kadomatsu emoji to a client and that could be completely acceptable, depending on the nature of the relationship. The Japanese, of course, would have additional formalities beyond a kadomatsu including year end cards, spring gifts, etc.