Awesome that they start now, super excited to see these and breathe a little cleaner air. Surprised that nobody has discussed the big picture of the timing yet:
Low scale production like semi trucks is easy to set up compared to the mass produced passenger cars. They could have built Semis for customers a while back if they wanted.
The reason for the hold up is that Semi has too high an opportunity cost vs other Tesla products. I posted a comparative price/kWh table
here in 2019. As you see in the table, Tesla had an order of magnitude more the revenue per kWh for their existing product mix compared to what Semi would have been. As battery costs go down, the general relationship of finished product revenue per kWh remains, which means that Semi won't make financial sense to produce until battery costs approach 0, with Tesla's current business model.
But the business model is about to change. The real reason to start delivering is that Tesla now believes FSD to be within reach. That will be the profit source, wether sold as software or a transportation service.
A limited number of Semis will be delivered to customers now to collect data for FSD. Until FSD is ready, Semi production will be a trickle.
For the FSD skeptics on this forum who will disregard the idea, let's revisit this thread in a few years and see how it turned out
.