This depends on your dog's health status and what you are treating for. My dogs get a variety of treats during training. If I am training a new behavior, I use something human grade and very motivating. If it's an old behavior, I will mostly use praise but might occasionally use something healthy from the store. One of my shelties LOVES a home made treat called "Salmon Crack." (It's crack to dogs). :) Another dog prefers a healthy store bought treat. They all will eat anything, but the higher the motivator during training, the better results.
Carrots are a choice you can try. However, not all dogs like them, and many dogs find them not to be motivating at all - if you are treating for training. BTW, this is knowledge I have gained from years being a professional trainer and working with over 1,000 dogs. :)
I never give treats "just because." My dogs love to work as I've made training a game. So they love to do something to get their treats. :)
You will find that there is much debate and strong opinions on what to feed dogs. You will also find much of the information isn't good. Be vary wary of getting information on how to feed on the internet. I've been amazed at some of what is out there. If you go with a raw diet, that is fine, but be sure you have researched it beyond the internet first. If you go with a healthy kibble, that is fine too. Pass whatever you are going to feed by your vet. Don't feel bullied into feeding or treating something because someone on the internet tells you it is bad for your dog. If your research proves otherwise, your vet is happy with it and your dog thrives on it, then you're good. :)