Why is this important? An example:

Npubs you somehow interact with *might* use your inbox relays to reply to you, to make sure you get the message on a relay you can read from and not posting replies in the void.

NostrDVM framework for example now integrates the outbox model. Clients like Amethyst check for a DVMs inbox relays and sends a NIP90 request, e.g. to generate a feed, directly to these relays.

The DVM receives the request, does some magic and checks for the user's inbox relays to deliver the reply to relays the user then again can read from and the client shows the result.

If your inbox relays use paid services for which the DVM is not registered for, the relay will reject the response event and you won't get a reply.

You want others to be able to write to your inbox relays, and you can't assume others have a subscription on any relay you might pay for.

Same goes for private inbox relays with DMs.

I hope this helps understanding what kind of relays you should put in inbox relays.