The idea behind price discrimination is that some customers will still buy the same product if it is offered at a higher price, while others will not. By figuring out which is which and offering them different prices, you can make more profit. For instance Uber is known to charge higher rates to customers with low phone battery, because they are probably more desperate and would be more willing to pay.
If a restaurant knows you have an expensive phone, they know you can probably afford more expensive meals and won’t walk out if the prices are high. If you have a cheap phone, they might want to tone it down a little to avoid driving you away. They might be able to make more money by doing this.
Also you wouldn’t need the waiter involved you can just check the user agent if all ordering has to be done through phones, the whole process would be automatic.