What is the use of honorary kill slogans in dota2 battlefield?

That is just a special effect. It is a reward during TI4. If you buy Xiaoziben, you will receive this gift from V. Then you can configure it to achieve double kill when you play the game. After three kills, four kills, five kills, and special kills, special effects will appear at the top of the screen, mainly color and light effects. Of course, players who play the game together will also see it, but people who have not purchased Xiaoziben will not have this special effect unless someone else gives it to them.