The shape sells itself on sight. Wide handlebars curving back toward the rider, a frame that looks longer than it needs to be, a saddle sitting well behind the pedals rather than above them. All of that is doing real mechanical work, and most of it is aimed at one narrow use: short rides, on flat ground, in ordinary clothes, at a speed that lets you look around. Whether a given bike does that well for you is a question about your proportions, not about the finish on the fenders.
What the three defining features actually change
Swept-back bars rotate your wrists to something closer to their neutral position and bring the grips toward your hips, which raises the torso and shifts weight off the hands onto the saddle. A long wheelbase, meaning more distance between the front and rear axles, slows the steering response and reduces the tendency to pitch forward over the front wheel at low speed. The laid-back seat tube places your hips behind the bottom bracket, so you push the pedals forward rather than straight down. Taken together, these produce a rider who sits up, sees traffic, and steers with small inputs instead of leaning.
The tradeoff is honest and worth naming. That position gives up leverage. You cannot stand and drive the pedals effectively when your hips are behind them, and the upright torso catches wind that a forward rider does not. Neither penalty registers much below roughly ten to twelve miles an hour on level pavement, which is where these bikes spend nearly all their working life. The design is solving for comfort in the first two miles rather than efficiency in the twentieth.
Why under five miles is the honest range
A trip of a mile or two to a store, a school, or a train platform involves more starting and stopping than sustained pedaling, and the things that make those trips pleasant are visibility, easy mounting, a stable low-speed feel, and the ability to ride in whatever you already have on. Those are exactly what the upright geometry delivers. Past five miles, the wind resistance and the fixed hip angle start to accumulate, and riders who thought they wanted this shape often find they wanted a slightly more forward one. It is worth being blunt with yourself about the actual trips you take, because the shopping decision follows from that and not the reverse.
Terrain matters as much as distance. Flat is the operating assumption. On rolling ground the seated-only pedaling position becomes a real limitation, and the question of gearing turns from academic to urgent, which is treated in its own piece on this site about what happens when one gear meets a grade. Stopping deserves the same scrutiny, since the braking hardware on this category varies more than any other component, and the separate article here on coaster brakes, rim brakes and hand levers is the place to work that through before you commit.
Checking fit in a shop, or in a driveway
Stand over the top tube first, flat-footed, in the shoes you would ride in. You want clearance you can feel without thinking about it, because a bike you hesitate to dismount is a bike you will stop riding. Then sit on the saddle with the pedal at its lowest point and see whether your leg extends nearly straight with only a slight bend at the knee, and whether you can still touch the ground with your toes without sliding off the nose. Both of those cannot always be true at once on a step-through frame, and knowing which one the shop is quietly sacrificing tells you a great deal.
Then reach for the grips. Your shoulders should stay relaxed and your elbows should hold a soft bend; if you are stretching, the frame is too long for you regardless of what the size chart says, and a shorter stem only partly rescues it. Roll the bike slowly in a straight line, hands off nothing, and feel whether the steering wanders. Browsing a category page for Cruiser Bikes will show you a dozen frames with near-identical silhouettes and meaningfully different top tube lengths, which is why the photograph is the least useful part of the listing.
What the wrong choice actually costs
The cost of a poor fit is rarely the price of the bike. It is the replacement saddle, the shorter or taller stem, the second set of grips, the shop labor to fit all three, and in the worst case the return shipping and restocking on a bike that arrived boxed. A frame that is a size too large cannot be corrected by parts at all, and the money spent trying is money that would have covered the difference to the right frame outright. The Consumer Product Safety Commission is responsible for the federal requirements that govern how bicycles are sold and assembled in the United States, which is a useful reminder that a boxed bike is a partly finished product; the article here on what arrives in the box and what is left for you to finish covers the labor you are either buying or performing.
Spend the extra twenty minutes standing over three frames before you spend anything else. A bike that fits gets ridden, and a bike that gets ridden pays for itself in trips you would otherwise have driven.