Re: Isn't that what the watches with the numbered bezels are for?
Before dive computers, there were dive tables. Dive tables are simplified and eliminate most parameters. Because of this they are very, very conservative. And even then they won't help you predict when you need to start your ascent.
For shallow water recreational dives, it's no big deal. You are down there to see the pretty fish. You have no motivation to get close to the limit. For more challenging dives there is a huge difference in capability and time spent at the destination.