It's almost never going to be the case that a lobbyist will convince a politician to support a measure they wouldn't otherwise support. Politicians all have agendas and preconceived notions. The most important thing lobbyists do is help politicians filter through he universe of proposals and convince them that particular ones fit within their preconceived worldview and platform.
I think many people have a hard time even conceiving that politicians could support big companies without getting some sort of kickback or campaign contribution for doing so. But a politician's first, second, and third top priorities are jobs. Big companies control the jobs, and for better or worse what's bad for big companies is often, at least in the short term, bad for jobs.
And besides, if a politician passed a bad law thinking he did a great thing for society because corporate marketing was very convincing, does that make a difference?
As to your last point: what is a "bad law?" Does anybody really have any idea what works and what doesn't? Did the repeal of Glass-Steagall cause the 2008 crash? We really have no idea. Did NAFTA leave consumers better off? Who knows? Do minimum wages increase unemployment? Experts disagree. It's effectively impossible to determine what impact laws really have, especially on the economy. Laws and policies aren't judged as "good" or "bad" except in history books. They're judged based on whether they fit into ideological frameworks and prevailing narratives. And voters are part of that. They want politicians who will vote for laws that fit their preferred narrative.
The advertisement makes you want that car, or at least makes you aware that it exists, which influences your behavior in the future, when you are actually able to buy.
The lobbyist never knows if the investment of time and effort is successful until after the vote is cast, and has no recourse whatsoever if is cast the "wrong" way. The car company can't demand a refund for its advertising if not enough extra people want to buy their cars. Watching the ad does not obligate anyone to buy. Lobbying does not obligate anyone to vote in a particular way.
Of course, as the lobbying game is played in multiple iterations, it is certainly a winning strategy to "stay bought". If a lobbyist invests a lot of effort into getting you to change your mind, they will likely continue to do so if you voluntarily act to make that investment pay off every time. There doesn't need to be a legally recognizable quid pro quo. The implicit deal is that consistent cooperation will be rewarded consistently.
