To add on to the above.

A sticker cannot be attached to a sentence. An emoticon can be used in any part of a sentence or standalone. That's why stickers are huge in size.

A sticker is for reaction, an emoticon is to indicate emotions attached to the sentence. Emoticon too can be a reaction with lesser emphasis (lack of animation & smaller size).

How to use them - they have the same function. To express emotions in the absence of body language.

To Steam, it's just a way to monetise more cute stuff and collectibles while meeting new interweb trends.