The priests in the tabernacle had no need of a chair because their work never ended, but Jesus Christ, the Great High Priest, offered himself as a single sacrifice for sins, for all time (Hebrews 10:12). After his sacrifice was offered and accepted, he did what no other priest serving in the tabernacle had done before. He pulled up a chair and sat down.
In high school, I worked as a packer in a major Canadian book warehouse. When new orders would come down the line, I would pack them and then move them to the shipping line. The work never stopped because the orders never stopped coming in. Unless it was a designated break time, there was simply no sitting on the job – there couldn’t be because the items kept coming on to my line.
The Bible indicates that, for the priests serving in the tabernacle, there was no sitting on the job either. Consider the furnishings in the tabernacle. There was an altar, a large basin for washing, curtains, a table, an ark, and a lamp stand. Interestingly, there is no chair in the tabernacle. Dan Phillips observes that the reason for the absence of a chair in the tabernacle is the same reason I never had a chance to sit while working in the warehouse – the work never ended.