The brief answer is that if you know, or can "reasonably anticipate" legal action, you have to take steps to preserve evidence related to it.
So if Google has been sued about, let's say, how they negotiate Revenue Share Agreements (RSAs), then they need to 1) take active steps to ensure any records they have about RSAs are retained, including making sure they aren't caught by any document retention policies and 2) they need to preserve any future records that might be relevant. And they also need to do that if they can "reasonably anticipate" they might be sued, which means (at a minimum) that if someone sends them a formal note telling them that they're planning to sure about RSAs (or whatever) they need to start retaining evidence.
If all your communications are either unrecorded or are deleted after 24 hours, then when you get sued (or receive a notice indicating someone is preparing to sue) then:
1) You likely have almost no retained records you need to prevent being deleted.
2) And in fact you can very likely argue that it took you more than 24 hours to implement the legal hold, so actually all your existing recorded conversations will probably end up deleted.
3) But everything going forward will be covered. And since these lawsuits drag on for years, that's likely to be quite a lot of material. Unless you keep deleting most of them, of course. Which doesn't seem like a good plan, but I suppose we'll see how it works for Google.