You nailed it... Great explanation!

These networks, besides serving as an development base for those looking to implement new products at Bitcoin blockchain, are also very useful for those just learning. Its very useful for new users or even experienced ones to see in practice, with technical details: how transactions, wallet usage, scripting, and even interactions with smart contracts work, without any financials risk (in the case of sidechains or Bitcoin extensions).

In other words, its an excellent community supported tool that greatly reduces the barrier to entry for those just learning and wanting to gain more technical knowledge.

You mentioned about Signet... did you know it can be customized? You can create your own private Signet with validation and signature rules that are completely under your control. I see this as being very useful for companies, for example, and even universities that need a realistic but also controlled environment to validate changes or even simulate network attacks and thus develop security measures.

Anyway... I think that for most people this goes unnoticed, but it's really good to at least know everything we have at our disposal.