Service entrance will generally have a main breaker.
Sub panels often don't have a main breaker.
Actually, adding a main breaker is often due to cost.
If the code requires a disconnecting means or overload/over current protection, a main breaker is often cheaper that a stand-alone
disconnect/protection.

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Ohm's law
Not just a good idea;
It's the LAW!

With the case of "sub-panels" in the same building as the feeder OCPD, that OCPD provides the overcurrent protection that is required by 408.36 for the panel. In those cases, it is a design and/or cost issue.

In some locations there is a requirement for a main disconnect, outside of the building, so a box with a main only may be specified, and on the inside the box would not have a main.
Or as said above, sub panels would not have a main.

Actually, if you really looked at the curves, you may not want a main, but opt for a fuse as a main.

Also, if you use a stepdown transformer within the building, such as a 480 to 240, you may have a breaker on the high side, and you may not want a main on the low side.

In most homes the builder will install a panel with a main, as this cost's less than having that in a separate box.

@cranky108
While the transformer may not required an OCPD on the secondary side, the secondary conductors will almost always need an OCPD per 240.21(C).

While the transformer may not required an OCPD on the secondary side, the primary disconnect is seldom accessible to the consumer so even if overcurrent protection is not needed, a disconnect is needed.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

Having done a heck of a lot of commercial design in a former life (over 1,000,000 sf of office space for just one client was a small bit of it) pretty much every branch panelboard in that realm is fed from a bigger distribution board upstream.

So, if this 225 panelboard is fed from a 225A breaker in that distribution board (that might be only a few feet away, why would I put in a 225A main breaker instead of going with a 225A MLO panel? If there’s ever a fault that needs to be cleared by tripping at the 225A level isn’t one breaker enough? If there’s two in series and the fault is after the second one they’ll both trip anyway, isn’t one enough?

I’ll see your silver lining and raise you two black clouds. - Protection Operations

I agree David. However there are cases where a visual disconnect may be required, and to some people a breaker is easer than a pull out plug. Mainly in times when the local distributer does not have any visual disconnect devices, and customer wants there service right now.

I have something like that on my well pump. A breaker in the panel, and a box with a breaker near the pump house (underground vault).

I agree with Cranky108, sometimes it is a matter of convenience. I just did that sort of project at a large plant that has a long long processing line. The MDP was at one end, my project was at the other end, it was about 1/3 mile between them. I put in a technically redundant 400A MCB in the local panel so that the electricians don't have to take the time to go all the way back to the MDP if they need to add a branch breaker to it. It COULD have been a 400A non-fused disconnect, but then I would have had to deal with SCCR issues etc. and besides, adding the 400A MCB to the panel was less work for the installer and less expensive all around.

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

That example is a perfectly good use of a panel with a main breaker. But I've done a lot of projects where an electrical room would have a main distribution board with multiple 225A breakers each feeding a 225A MLO panel in the same room. No need to waste the money or the panel space to have a main breaker within reach of the same sized breaker feeding it. Do what makes sense for the project.

I’ll see your silver lining and raise you two black clouds. - Protection Operations