Part of it is about incentive alignment. There's about 100 ways to make food 1% tastier with 5% more calories. Restaurants make money from food that is tasty and perceived as healthy, regardless of whether it's healthy or not so they tend to utilize the full arsenal of tricks.

When you have a chef in house, they're being paid to make the food to your specifications so it can often be significantly healthier.

The benefits of chef-cooked food (vs. delivery) are more subjective, but I can see situations in which an argument could be made for a net benefit to the startup. If you make an effort to restrict yourself to healthy food (along with other "healthy" lifestyle choices, such as consistent and sufficient sleep), you can realize significant productivity benefits in the types of white collar work that being part of a startup typically entails.