First, let me reference the perfect succinct articulation of what the difference is between on premise and off premise catering:
On premise catering happens when your kitchen is within the venue where an event is held. The food is prepared on site and served there as well. Off premise catering involves a kitchen at another location. The prep work is all done at the kitchen and is transported and completed at the venue OR is completely cooked at the kitchen, plated, covered and transferred via company trucks. Once there, the food is served or set up. Think anything from Subway delivering foot longs to a corporate lunch to an expensive caterer bringing your food to your wedding for offsite. Usually event halls and other venues where banquets and weddings are often held have offsite catering. Restaurants, country clubs, etc that routinely serve food (outside of events) have onsite catering. – Kaylee Straub
My first ever event-related job was at an event venue (it was also a nightclub on the weekends) which had off premise catering. I actually felt that doing events with off premise catering was even easier, in some ways, than having your chef in house. But when I finally moved to a hotel with on-premise catering, I became much more highly involved in the detailed menu planning and pricing that I didn’t deal with before.