There can be multiple reasons for both hiring and not hiring a restaurant marketing agency.
I will try to give reasons for both hiring or not.
Some scenarios you might not even need to hire an agency, while in other you have to.
You don’t have all the expertise so you need someone dedicated.
You need consistent flow of customers and want to grow your brand like opening multiple branches, increase in restaurant size etc.
Someone to manage social media, take photographs/videos every now and then post it with your style and theme.
Now, when you marketing agency, it also takes care of many areas like designing, videos, photography, website maintenance and updates, etc. Make sure you get scope of work sorted.
For not hiring:
Now, the answer to why you don’t need restaurant marketing agency.
Very small business like 1 person or a stall.
You have enough time to do basic marketing yourself.
You have great taste that drives returning customers.
Don’t want to grow too big too fast.
Reason#5. Lack of funds.
Hope it helps.