That's a great question, and one I've wondered about myself while walking past them. The short answer is: they help, but they're not a magic bullet.
On the plus side, these combo units address a specific need. Smokers often don't know what to do with their cigarette butts, especially in areas where ground littering is fined. Providing a designated, hot surface for extinguishing and disposal right on top of a bin gives them a convenient and responsible option. It signals that the city has considered their behavior, which can encourage compliance. You'll often see butts in these trays, proving they get used.
However, their effectiveness has limits. If the ashtray compartment is full, overflowing, or rarely emptied, people will just toss butts on the ground next to the bin anyway. Their success also heavily depends on placement. A can outside a busy office or bar will see more use than one on a quiet residential street. Furthermore, some dedicated smokers might prefer standalone, wall-mounted ashtrays that are more obvious and easier to tap their ash into.
So, are they helpful? Absolutely. They capture a significant number of butts that would otherwise become street litter. But for them to work well, they must be part of a broader strategy: regular maintenance, strategic placement, and clear public messaging. They're a useful tool in the toolbox, but not the complete solution to smoking-related litter.