If you were to just use it without diluting it, you would be wasting the product by using a version that probably won't be any better without the dilution.
Sometimes you may feel like the product quality isn't a big deal but when you consider how much better some things work than others, it makes sense to save a little extra so you can afford the best of the best.
If it needs to sit in the drain for a while but it will also eat through the pipes, that can lead to damages that are expensive to deal with.
If a pet were to get into your tub and try to drink out of a drain that wasn't clean of chemicals, it could do some harm to your pet so you have to be careful about this.
If you're not sure if you should use one or not, ask a professional to see what they think you should do in the situation.
There is a reason why top of the line plumbing services use only the best known brands when it comes to chemicals.