Seat number

I sometime wonder why train company bother attributing you a seat. As usual, when I was ready to get to my seat in the Thalys, I found it occupied.
Me:"Excuse me, which seat do you have?"
The man sitting at my place: "43"
Me: "well I have 41, and that's 41, so..."
Him: "yes, but someone is sitting at my seat, so why don't you take one of the free ones over there?"
Me: "Because I booked the seat by the window without neighbour. Why don't you get your seat"
Him: "It's only a seat, don't give me the attitude and just go sit somewhere else!"
It was Saturday morning, I didn't want to make a scene so I grabbed the someone from the train staff, asked her to get the man out of my seat which generated a mega domino effect because somehow almost no one was at his seat...
Now, why do they bother attributing us a seat if no one follows it???
PS: after that trip went very smoothly and I enjoyed my breakfast watching CSI :)