When purchasing a Painting/Print/Sculpture/etc., you're purchasing something that compliments the room, maybe completes it. You want to find something the right size, color, style...something that says "This room just needed this."
When purchasing Artwork, you're truly purchasing a piece of the Artist. Something about the Artist captures you, needles at you to say "I must have this creation." Maybe it's the Artist's style, maybe you witnessed the creation of a particular piece and wish a keepsake of that moment in time, maybe there's an emotion provoked and you just have to have it. Sometimes you quite literally purchase a piece of the Artist in one of his works - the blood, sweat, tears that went into it.
Decorations compliment. Art captures.