A few reasons.

1. I’m a dev, not a lemmy dev, but a web developer. I spend a lot of time thinking about difficult problems. Blocking an instance is one of those difficult problems because while it can be done it’s tricky to do it well. Connect for lemmy does this, and when I block an instance, I don’t see posts from that instance, and comments are hidden behind a message that says the comment is from a blocked instance - if I tap that message, I read the comment. This may not fit the definition of “blocked” by some people’s standards, as it’s more of a “hide” feature. So, I’m somewhat interested from that aspect.

2. I use the feature myself mostly for testing, but I do wonder if the idea of blocking instances because we don’t like what we see is an appropriate idea to begin with. It’s like saying “block gmail.com” because you have a few people on there you don’t want to get emails from. Very possible to do, but it seems there’s a few thousand more people who might also be worth emailing and you’d miss out if you blocked them. Also, as we taylor the internet to smaller and smaller peices, we decrease the rate at which we find new content. This is fine if if that’s the goal, but to block whole instances and then later complain about lemmy dying because no one posts content on your instances would be weird. Not that you’re doing that, but it seems to be the next logical step if the idea of blocking instances picked up more steam amongst lemmy clients. As others have pointed out, it’s probably best to set your language prefs instead, as this is the mechanism the devs likely put in place for speakers of x language to avoid languages they don’t know.

But yeah, google translate exists, not everyone on one instance may speak only that language, etc. But it’s your internet experience, I ain’t gonna knock it.