It's no doubt that being a professional singer requires a certain extent of innate ability/talent. But even with that talent, does that singer need to practice every day? If yes, what is the purpose of practicing and what are some kinds of exercises that a singer should focus on?
This varies from one person to another. In the cello world, Rostropovich was famous for not having to practice much. But most cellists do need to practice almost every day. Extrapolating, I imagine that some rare singers would be the rare bird like Rostropovich. For vocalises that make sense for you, please consult a voice teacher you trust.
Use it or lose it. An oft quoted saying. Just as true of an athlete, as a vocalist, as a surgeon. Whether one would lose one's talent in a day is debatable, therefore a matter of opinion, therefore not something acceptable on this site.
However, most professional singers would not let many days go by without singing something, even if they were cooking their meal, or better still, having a bath! It's not like they need to go somewhere to practise without bothering others, or set up their instrument, etc.
Professional singers will be singing at venues, or rehearsing with accompanists anyway, so there may well be not many times when they're not singing. On those odd occasions when nothing much is happening, they'll probably do some warm-up exercises - scales, making strange noises, at least keeping their singing muscles and other body parts supple. Why wouldn't they? Certainly when a concert or such like is imminent, there would be lots of singing.
Then there's the learning of lines of words. Why try to learn them as if speaking, when they need the melody? Singing also incorporates breathing techniques, which will go slack in no time, so practising daily seems to be a great way to keep things on the boil. Athletes need to (are expected to) hone their skills on a daily basis, why not singers? I know trumpet players who lose their embouchure after only a few days of not playing, and used to coach a world champion water skier who came back from a week's holiday with his edge quite lost. Use it or lose it!
As far as exercises are concerned - as with choirs, a warm-up is a good thing to do, if nothing else. And looking at new material, not necessarily polishing it, could come after. But all this will depend a lot on how far away the next gig is...