What does black achieve with 1.d4 c6? I have faced 1...c6 as a reply to my 1.d4 quite a few times recently, and this makes me wonder why people play this as oppose to 1...d5.

In addition to playing the Slav with 2.c4, I have an option of playing the Caro-Kann with 2.e4, so I have an extra option and black has nothing in return that I can see.

1...c6 happens at top level, but fairly rarely (9th most popular response, behind 1...Nc6 according to lichess database).

Why do some players choose this move order?
You're right that players who choose 1...c6 must be fine with the Caro-Kann, which is one reason why it's not that popular. However, there are some people who are fine with the Caro-Kann, and so the move gets played occasionally. In the case of 2.c4, there aren't many benefits I can see for Black. He has the option of playing a la King's Indian with ...Nf6, ...d6, etc, but it wasn't necessary to start with 1...c6 to do this.

There may be some minor benefits to 1...c6, but they're not obvious to me. This is why the move isn't played often.