Why facebook have tor hidden service?

Possibly three reasons.

1. As suggested, access beyond the clearnet where Facebook is blocked;

2. One point may also be script usage. You commented that users can access Facebook through Tor, but one of the drawbacks of Tor Browser is its performance with scripted web services, particularly webmail and the like, rendering many websites and web services unusable - there are many examples. This has an impact on Tor's value as a speech vehicle, depending on where you want to speak: signing up for the average forum involves getting an email address, opening a forum account, responding to confirmation email, and then posting, on the clearnet widely involving the use of javascripts and requiring java to be enabled in Tor. I personally avoid this experience, and the pain of sitting through crosswalks and bicycles over and over again and watching websites fail. By this point, much of the value of pseudonymous access seems lost. Accessing the service as an onion service might avoid this - though script enablement is a typical channel for deobfuscation attacks;

3. Providing a service as an onion address allows Tor to be used to access it without exit nodes at all.

Onion services aren't just about hiding things. They are about providing a platform for users who need to be anonymous.

The fact is that many countries block Facebook but Tor users are still able to access it via the onion service.

See Facebook's website here and here for more details.