Yoshida is the one who represents Public Safety in the story, trying to keep Denji from turning into Chainsaw Man and setting up their interest in Chainsaw Man erasing the Death Devil during the Aquarium arc. It’s his talk with Death that introduces the Nostradamus Prophecy into the story and leads into Falling’s first appearance.
He walks Denji through what’s happening in the Church arc and makes the ultimate consequences clear to him, and then following through after the Fire Devil riots. It’s his fight with Asa and Yoru in the apartment where we first see how Yoru is becoming more powerful as fear of War spreads across the globe.
In the Aging arc, Yoshida is the one who gets Pochita to vomit up Ear Devil and thus reveals the machinations of the erasure ability, as well as formulating the plan to escape Aging’s world.
Whether you care for the character or not, he’s done plenty to entrench himself within the plot.
Reze and Yuko’s significance doesn’t really work as a comparison due to the differences in their respective characters and arcs.