One of the functions of the article ὁ is anaphoric. It points back to a previous reference. A simple but clear example is in John 1:7.
John introduces the topic of Jesus being light in v 4. "In him was life, and that life was the light of all mankind" (NIV). It occurs again in vv 5 and 7–9. The problem is that most translations start a new paragraph at v 6 when John is introduced. Thankfully, there generally is not a heading here, just a new paragraph.
Here is the problem. In this new paragraph, v 7 reads, "He came as a witness, to bear witness about the light (περὶ τοῦ φωτός), that all might believe through him" (ESV, and most). If you have been reading the larger unit, vv 1ff., the identity of the light is clear — Jesus. But if you start reading at the new paragraph, or start preaching with that paragraph, then the identity of the light is less clear.
I assume this is why the NIV reads, "He came as a witness to testify concerning that light." They see the issue and translate the article explicitly as anaphoric, which it clearly is, and so say "that" and not "the." It appears they are the only translation to do so.
By the way, there was an anaphoric article in v 4 as well. Did you catch it? In him was life (ζωή), and that life (ἡ ζωή) was the light of all mankind" (NIV, also CSB).
This is just a little example of the anaphoric function of the article, and why a translation with a sensitivity to literary style and to how the text might be read adjusts for those concerns.
Of course, if we would get rid of typographical conventions exegesis would be greatly helped. I remember the first time I saw the gospel of Mark without chapters, paragraphs, and verse numbers. We referred to specific places with page and line numbers. That's when the basic structure and message of Mark came alive for me and why it is still my favorite gospel.