Putin has many people killed quietly with staged car accidents or heart attacks or even cancer.
Once in a while though, Putin wants to send a clear message to the entire world that he can and will murder those who oppose him. When he needs to do this, he will use rare poisons only accessible to him as the leader of a large nation.
The poisoning of Navalny with a nerve agent is one of those cases.
Honesty edit… I have no proof that Putin kills people quietly. It’s just a theory based on my opinion of how his brain works. We do have evidence he does public poisonings as he makes it very clear he did it by using rare poisons.