If my roundhouse makes you guard your head more (so no others land) then the defense somewhere else is somewhat lessened. In general, a testing kick (one with less power) is used when you have little expectation of it landing, but want to see what the defense is. It's like a quick jab, which also has little chance of doing damage in a single strike.And you may not get a second chance to land that same kick. Kick me too light in the head and I may prevent all other round house kicks from reaching my head. That means the only round house kick that landed was actually wasted. This is the mentality of why people say (Make every shot count).
I understand this is not the case for many but I think from the perspective that you and I often look at martial arts, It' just better not to do things that won't cause harm to your attacker. In sports, it's going to be different. In training it's going to be different. If a fight full contact like MMA or a street fight, everything should count. Things like Jabs and snap kicks should be hard enough that your attacker fears it.