Although not enforced, it’s always recommended and here’s why…
Your hired vendors (photographer, videographer, DJ, etc.) are with you all day and so they will be hungry later, just like you will be at your event. Feeding your vendors, simply put, is a nice thing to do because it shows them you respect their job position and that you want them to be comfortable of which they in return, will want to do a especially good job for you.
Keep in mind, it’s not recommended to put vendors in a position that they need to leave the premises to purchase their own food elsewhere while the event is active. After all, they are there to perform their job and need to be present to make sure everything is running smoothly. Especially to catch any unexpected joyous moment that can be lost forever, just because they were not present? Would be surely a disappointment, something we all know that no one wants.
This is why it is always recommended to take care of your vendors by inviting them to join in.
(Of course, anyone would understand should your budget not allow it but kindly informing your vendors ahead of time prepares them to make other arrangements.)