They’re actually providing you different metrics to let you take different actions. You might be interested in click rate to know, for example, how many people out of your ENTIRE email send clicked a link: “Oh, I emailed my whole customer list, and 10% of my customers clicked a link.” You might not care at all what the clickthrough rate is, in this example -- just that 10% of your customers clicked something.
Clickthrough rate, on the other hand, tells you how many people clicked the link *only* out of those that opened it. Thus, if your click rate was low but your clickthrough rate was high, you would know that not many people opened the email (and check the open rate to validate that knowledge), but for those who did, it was very effective in getting them to click -- so you would know for future campaigns that your body copy was effective in driving clicks, but maybe your subject line (for example) needed work to get more people to open.
These are just examples of why these two metrics exist, and why they might be important to different people for different reasons. Hope it’s helpful to someone, either now or to someone who might find this thread in the future 