I don't think the market will care about the Tesla Semi until it's sales are contributing noticeably to the bottom line. Until then it's likely just a talking point to Wall Street.
For us long term investors it's very exciting though, as we all know how bullish the Semi will be long term.
I think the Tesla Semi is important to the investor mindset.
So many people have said it violates the laws of physics and/or will never happen, including Bill Gates as recently as 2020, that many investors have serious doubts that Elon knows what he's doing. To people that don't know any better, people who can't do simple math on the energy required for a semi and the energy density of batteries, or don't want to bother, this makes them think Elon might be a fool, a dreamer, or worse, a shyster. They think it's release was delayed because it's just a pipe dream, a mirage created to snare gullible investors.
Delivering even one semi that can demonstrate it really is real, and doesn't need a route that is all downhill, will make them question all their other doubts about Elon and Tesla. It will be one more "lie" that has been proven wrong. How many more Elon fantasies or "lies" are actually the truth? Cybertruck? FSD? 20 million cars by 2030? A path to being worth more than Aramco and Apple combined?
Every new milestone, every promise fulfilled, helps Tesla gain respect and a few more closet TSLAQ to see Tesla in a different light. I'm not saying the Semi event alone will cause an instant rally, but the market does "care" about credibility and respect, both of which are earned, not assumed. Following through on your announcements is how respect and credibility is earned. We knew this day would come, but not everyone did.