One thing I do quite often is, when entering a marriage, I will be editing both profiles at the same time, in different tabs. I copy and paste the text from one profile to another, and then I want to add a link to the other party in the text of the biography. To do that, I add the person's ID to their name in the text, add square brackets, and I'm done. (Seems a little ridiculous to be explaining this to you :-) ).
It's not impossible to find the text elsewhere, for example in the little popups that appear over person links, but that is actually quite tricky to work with, especially on a tablet, which is my usual editing environment. The URL bar on the other hand is quite an easy target for selecting and copying text - and the ID I wanted used to be at (or sometimes close to) the end of the URL; and it's free of stray punctuation etc.