Why do they still have guide dogs in the future? They typically only last for 6-7 years, and it's not like anyone would be training them after the apocalypse. The two seen at the end of the movie must have been trained right before the apocalypse, and even then they would be pretty old by dog standards.
On the contrary, a seeing-eye dog would be an incredibly valued companion in a world where most humans can't safely see the world around them. Training new seeing-eye dogs would be a major benefit to a community. From what we see, humans are the only animals we know of that are affected.
The question posed by the OP is not about why people would use guide dogs; rather, they are questioning how anyone could possibly train them in the first place when they (the people) are unable to see.
It's not a big stretch to imagine someone there with sight has a degree of familiarity with training guide dogs. A member of staff, a friend of a staffer or student, a professional colleague who realized it would be either a good place to seek a community or felt obliged to help. Or even someone familiar with dog handling in general, and learning the specifics from information in the school and people who are taking refuge there. We've used dogs this way for hundreds of years, so even if modern training techniques aren't something we can continue we can probably get them trained well enough.
Companionship is valuable in a world where the human population is dwindling quickly and the remaining humans are isolated from each other in order to survive.
The title symbolically relates to the main action of the book: the children "leaving their nest" for the first time and taking a major step in learning how to function on their own.
The creatures can be a metaphor for the truth about oneself. When people look at them they see their true self and all their trauma reflected in them. In another words, the truth is so devastating, that they commit suicide to not admit to it or even allow to expand their perception of their reality.
We never find out, nor is it considered by the characters, what would happen if you physically restrained someone from committing suicide after they'd seen the creatures? Would they snap out of it after being brought back to safety, or are they forever doomed to keep trying to kill themselves?
The book tells you exactly what happens - they still find a way to kill themselves. It's not pretty. He's described as having turned himself into "cake."
In the same vein, the movie invites the viewer to consider just what would happen to a newborn infant who saw one if their parents didn't. We never find out, but given the inability of a child that young to effectively kill themselves and the fact that all of their real mental development is ahead of them, it's difficult to imagine just how differently they would develop with that unremembered image in the back of their mind. It's also difficult to imagine that the direction they took would be good.
Despite the relief at the end, humanity is still screwed. They really have no way to combat the things, so the future is at best a few isolated groups of humans hiding inside forever.
Except if these humans learn to be authentic and instead of rejecting their trauma, they start to feel and resolve them in the process.
Food won't last forever. Pre-packaged food will eventually expire or run out. Farming and foraging blind is a big challenge (not to say it can't be done blind, but there are technical issues) and you can't have the sighted people calling out over a radio/voice guiding the blind folks.
If dogs can be affected by the creatures, what about other animals? Many species would be entirely wiped out, and it will affect many of Earth's ecosystems.