Whenever you enter the US, you are required to fill out Form 6059B, which asks for your personal details and whether or not you're bringing in any restricted goods. Whenever you are entering the UK, you are required to fill out a landing card and passengers entering Canada fill out an E311 Declaration Card. As far as I can tell the immigration officers usually completely ignore whatever details you wrote in those forms and simply scan your passport to enter your details into their computer. It therefore looks like a complete waste of time, as the majority of passengers have no goods to declare anyway and therefore the immigration forms get thrown away right after you get your passport stamp.
So what's the point of these forms/declarations? Why require absolutely everyone to fill them out? As a bonus question explain why car passengers are spared from this obligation and are instead asked verbally if they have something to declare.
Update: no more landing cards in the UK and US! Now just waiting for Canada to change their outdated practices.