When to use Seneca?

Everything you've said in your question is correct- there are obvious situations where you want to play one of the other HQ types. This is rarely the case for Expansive.

The advantage of Expansive is that they're extremely versatile and adaptive. Their faster, cheaper transports mean they can acquire raw resources from farther away than other factions.They get extra claims (for a total of 22 instead of 18 at level 5), which means they can invest in a wider array of buildings and/or use more specialist buildings. Their reduced steel costs make production buildings much cheaper to replace, so when a resource does spike/crash, they can be the first to profit from it.

The ideal map for Expansive is one with plenty of resources, with high deposits, that are reasonably spread out. Ideally you don't want Bribe a Claim available- it benefits your rivals more than you.

Alternatively, Expansive allows you to work with less specific starting locations, which can help if your first choice gets taken or you want to found with less debt. You don't need a massive amount of carbon like a Scavenger, a triple iron and large area of water/silicon like a Scientist, or close resources for your adjecency bonuses like a robot- just access to the resources you may want.