Most studios draw a beautiful website and hand the files to a separate development shop to build. The seams between those two teams are where projects quietly fail: the design that could not quite be built, the interaction that got value-engineered out, the launch that slipped because each side was waiting on the other. Putting design and build in one team removes the seam. That is the whole point, and it changes more than it sounds.
Here is what the seam looks like in practice. A designer specifies a smooth page transition, the separate developer says it is out of scope or too slow, and three weeks later you approve a build that feels nothing like the mockup you signed off. Nobody lied. The design just assumed things the build could not honour, and there was no one who owned both. When the people who decide how it looks are the people who make it work, that gap closes before it is ever drawn.
The obvious worry is that one team costs more or moves slower. In our experience it is the opposite on both counts. Fewer handoffs means fewer review cycles, fewer rounds of "that is not what we meant", and no time lost translating a design file into a written spec for someone who was not in the room. You are paying for one conversation instead of managing two vendors who blame each other when something is off.
If you already have a developer you trust, you do not need to replace them. What you want from a studio in that case is design that arrives as a real, buildable spec: components, states, and edge cases, not just pretty screens. Ask for that in writing. The trouble only starts when a studio hands over static mockups and treats the rest as someone else's problem.
Plenty of studios say "we do development too" and then quietly subcontract it, which puts the seam right back. To check before you sign, ask three things: who actually writes the code, whether you can see recent work they built and shipped rather than only designed, and who you will talk to when something breaks after launch. If the answer to that last one is vague, the gap is still there, just hidden from you until it matters.
The honest trade with one team is capacity. We cannot run ten projects at once, so we take fewer and turn down work that only wants a picture with no intention of shipping. If you need a large agency with account managers and a war room, that is not us. If you want the people doing the work to be the people you talk to, it is.
So when you are choosing, ask each candidate the plain question: who actually builds the thing. If the answer is "someone else", you are the one who inherits the gap between them. Handing over the whole problem, design and code together, is usually cheaper and a great deal calmer than stitching two vendors together later.