I'm a dev, not a salesperson, but let's be realistic. A company tells you "yeah we're interested in signing at $1M/yr, but we really need this feature, when will you have it by?", to which saying "eh we don't know - it'll be done when it's done" will lead to the company saying "ok well reach out when you have it, we can talk again then" (or just "eh ok then not a good fit sorry bye"), and in the meantime they'll go shopping around and may end up signing with someone else.
Having a promised date lets you keep the opportunity going and in some cases can even let you sign them there and then - you sign them under the condition that feature X will be in the app by date Y. That's waaaay better for business, even if it's tougher for engineers.