Why should I use soap and water to wash my hands? Germs can get onto your hands and items you touch throughout the day. When your hands may be dirty, it’s best to wash with soap and water to remove whatever germs and chemicals may be on them. Warm or cold water? Either is fine, as long as it’s clean. Bar soap or liquid? Either is fine. Does the soap have to be antibacterial to work? No. Plain …

It's seem to be a pure matter of nuanced semantics, but it's quite a fundamental difference. it's best to understand it as we understand insurance in a more traditional scenario: for cars. 

We don't want to get into accidents. And more often than not, we are rather attached to personal, fashionable belongings such as our cars—we name them; we spend a bunch of time in them; etc. Point is, we try to avoid using the car insurance that protects us from financial ruin when we [knock on wood] get into a car accident. 

We don't want to use our car insurance. 

Same is the case for health insurance—somewhat. It's supposed to work like this. It's there to protect us from financial ruin in case we get gravely ill. We're not supposed to want to exercise on those benefits, and yet, soon as we get us some good ol' company sponsored health insurance: we're like kids in a candy store, and the first thing we try to do is max out our benefits: seeing all kinds of doctors, seeing if we can get whatever tests done that we can get ordered. 

First off— Do you know what the difference is between health insurance and a health plan? There's a fundamental difference, and if you're not totally aware of it, read this. 

So, being aware of that difference, you know that there's an incentive for insurance companies to not have their subscribers utilize their benefits. It's like car insurance— You're not really incentivized to use it; i.e. you don't really want to get into an accident and have to utilize your car insurance benefits. 

Same with health insurance— You don't really want to get sick: But it does happen. When it does, and you come in for a "sick visit," you have to pay a co-pay (your financial responsibility as a subscriber). 

But— When you go in for an annual physical, or if you go in to see the doc for shots or something preventative: Now, that's a different story. That helps insurance companies. That helps them because that helps you stay healthy for a longer period— i.e. you won't have to "cash in" (utilize) those health insurance benefits (i.e. sick visits / visits to the doctor for when you present with a clinical symptom). 

A co-pay is a kind of financial responsibility. Most health insurances and even supplemental and government sponsored health plans obligate the member or patient to some kind or some degree of financial responsibility. Co-pay requirements will vary depending on the type of benefit that the health insurance is currently called to cover— for example, many health insurance HMO products do not charge a patient a co-pay when the insurance subscriber utilizes a kind of preventative care benefit— such as seeing the doctor for an annual physical.

Want to know why? See: "Why health insurances don't usually charge co-pays for preventative care."

See also:

What is a co-pay? 

What is a share-of-cost?