An extremely important and very rich member of the nobility, Lord Steyne becomes Becky's debauched and cynical entrée into the highest peaks of English society and government. She is with him for fame, status, and money. He is with her for extramarital sex. Does she actually sleep with him? We will never know.
Most of Thackeray's characters mock their real-life counterparts with a gentle sigh. But Lord Steyne is a giant, pulsing red finger of accusation aimed straight at the highest ranks of British aristocracy. He's also a pair of eyes rolling at the reader in disdain and disgust. (Why, yes, that is a lot of disconnected body parts. Shmoop is all about dismemberment, metaphorically speaking of course.)
In describing this awful man, Thackeray is saying two things.
Thing one: are you kidding me with this guy? How on earth do we (the 19th-century English people who were Thackeray's readers) live in a society where the likes of this degenerate can climb to the highest pinnacles of political and economic power just because they happen to inherit titles? Do we want these people in power simply because of their wealth – or should the contents of their character have anything to do with their position?
Thing two: are you kidding me with how much everyone sucks up to guys like this? Don't get too comfortable, readers (well, 19th-century readers, but if you take out the nobility part this still applies). Sure it's easy to sit back and condemn this invented Lord you're reading about, but think about how you would behave if he invited you to dinner. Or if he wanted to patronize your place of business. Admit it, you'd kiss his rear end just as much as the next guy. Is that something you're proud of, or would you rather it weren't the case? Is there a way to transform society so this kind of currying favor based on status weren't necessary for economic and professional success?