The Slav is a way to protect the pawn-d5 without many of the disadvantages known from the QGD. The Bc8 doesn’t get imprisoned, the exchange on d5 doesn’t result in a Carlsbad-structure and last but not least, there is no Bg5-pin.
The first logical try for white is the Exchange-Variation. If black just copies the white moves, he will lose by force. This was enough justification to play the line in the old days. The good news for black is that he can deviate with 8… Nd7, where he is just fine.
Since the Exchange-Variation doesn’t seem to promise much, white should try to interfere with the black ideal setup, which is posting the Bc8 outside the pawn chain. Simple development with 3. Nc3 prevents Bf5, but it allows a few other options like the Noteboom. As a rule, whenever there is a white knight on c3, the lines with taking on c4 become more playable for black, because a later b5-b4 comes with tempo.
Note: The Triangle allows white to play the Marshall-Gambit which contains a few forced draws.
To avoid all of this trickery, white usually starts with 3. Nf3. The following sequence is supposed to be the best move-order:
As we can see, white does not allow the smooth development of the Bc8. On the other hand, black can always find a very concrete way to deal with his problems.