This post is about the history of Safari. If you only have one (only personally accessible) computer, then you probably don’t have to worry too much about it. Browsers have the ability to clear history and other data under the hood. My browser is Chrome, it can do that too: I recommend all Chrome users to take a look at Clear Cache. This allows you to set very precisely what is deleted – and from what period of time.
With Safari, clearing history is always a thing. Although you can delete the history of the last hour, the current day, today and yesterday and completely everything, you will also lose cookies. This can be seen when you select History > Clear History in Safari.
However, these cookies also specify where and how you logged on. So if you delete everything, you have to log in again on all pages. That can be annoying. And that not only affects the current computer but also, if available, iPhone and/or iPad. See the screenshot above this paragraph.
But how is everything preserved – except for the history? If you open the Safari “History” menu, then hold down the Alt/Option key while doing so – the menu will change and say “Clear history and keep website data”. So you delete the history on all devices, but cookies – and thus also login information – are retained.