Fresh food can genuinely help with weight loss since it's usually more satiating than kibble, so your vet's onto something there. The main thing to watch is that your dog's digestive system needs time to adjust - rushing into it tends to cause soft stools or worse. Mix it in gradually with the old food over about a week or so, starting maybe 25% fresh and working up. Beyond that, keep an eye on your dog's energy and coat condition over the first month or two; some dogs thrive on it while others do fine staying on kibble.
The cost is real though. Depending on your lab's size and how much they eat, you're probably looking at noticeably more per month than standard kibble. Some people find it worth it if their dog loses weight and seems healthier overall, but it's not magic - you still need to watch portions. A lot of folks also end up doing a mix, like fresh food for some meals and kibble for others, which takes the edge off the expense while still giving some of those benefits. If you want to test the waters without committing to a subscription, many of these brands sell smaller packs in stores these days, so you can see how your lab actually responds before going all-in.