Third-party Google Assistant devices can add custom commands now thanks to Google enabling the option for manufacturers who make those products. Ever since Google Assistant has been available in any of its forms across the various devices which it's now been integrated into, there have been just a couple of commands available to activate said devices, even starting with just one, "OK Google," before adding "Hey Google" not too long ago. That isn't really changing, but brands who make Google Assistant products will be able to create custom commands for various functions of their products after the Assistant has been activated, which should make things feel a little bit more personalized.
For the brands, this is a big thing as each brand will have their own special offering in terms of features that will make it unique and differentiate it from other brands selling the same types of products, and with custom command support in place, each brand can add their own flavor of commands for features or functions that are specific to those products, as opposed to only being allowed commands that were universal to a product type or category. This is certainly good for consumers, but it's likely to be better for the brands as it will help them make their products more useful to the consumer with a veritable and unique set of Assistant commands and actions.
This could be really useful for home appliances which have tons of different settings, as manufacturers will have more freedom to add Google Assistant commands for all of or most of them. Ovens, coffee makers, microwaves. All of these now have the ability to have voice commands set up that would allow users to say things specific to those devices, like starting a pot of coffee or setting the defrost option. This could end up giving more control to the user in the end, moreover it enables manufacturers to add a complex set of voice commands to their products for extremely intricate Google Assistant integration. That level of complexity, too, could entice buyers to look more at their products as opposed to those from competitors as controlling it all through Google would be much simpler than not having the option.