The Alapin Variation is the rather radical looking response to 4...dxc4. The move cuts off the c4-pawn's lifeline but it leaves b4 weak and ripe for invasion.
This is a price White is willing to pay for a good center. Black's Queenside Knight often shows up on b4 sooner or later. Or maybe the dark square Bishop in some lines of the Czech Defense.
The Main Line shows White winning the battle for space in the center. You will also have play on the Kingside. As Black you get a solid, compact position with counter play through your prize on b4. You also have the thematic Slav/Semi-Slav breaks, ...c5 and ...e5, in reserve.
If you're not happy with White's positional assets in the Main Line, you can liven things up after 5...Bf5/5...e6 with 6.Ne5 instead of the more methodical 6.e3. Now things may get tactical as both sides see their centers liquidated for an open battle.