Example (Putin is actually in serious trouble):
Moscow has no doubt been very effective mounting guerrilla marketing campaigns aim at sowing doubt and confusion in the West. And they have been skilful in manipulating and surreptitiously influencing media narratives on issues like the Ukraine war and the downing of flight MH17.
Without that between campaigns and aim at, it's very difficult to keep up with the flow of the sentence when you read it fast because it's quite possible to read guerrilla marketing campaigns aim as one thing, one unit—a noun phrase, in other words—and then halfway through realize that something's up grammatically with what you just read. And then you'd have to step back to read it again and then mentally plug in a that to smooth out the grammar. Don't you think so? Do you think the sentence would sound better with that?