The Semi's 500-mile range is apparently well-suited to Walmart's usage habits, which revolve around one-day round trips. In other words, it wouldn't work so well for multi-day or multi-point trips where there wouldn't be many opportunities to recharge.

This isn't the hugest order (PepsiCo, for instance, has 100 on deck). It does represent a vote of confidence from a major brand, however, and that could be crucial to Tesla when it's still trying to win over customers ahead of the Semi's 2019 production start -- especially when competition is breathing down its neck.