Steel bottom bracket shells come in lugged and lugless varieties. Lugless bottom bracket shells are just tubes that have been threaded and machined to accept bottom bracket cups (or bored and reamed to accept press fits). Lugged bottom bracket shells can be cast, stamped or hydroformed to have sockets to accept the seat tube, down tube and chainstays.
The big difference is generally considered to be aesthetics, despite being hidden behind the crank. Lugged bottom brackets also limit your geometry angles, tube sizes and bottom bracket compatibility. Lugless bottom bracket shells are made in a wide variety to widths, threadings, pressfit bores, etc and have no restriction on what tubing they must be mated with or limitation on what geometry must be used. Lugless is also usually much cheaper and a fair bit lighter, and there are examples of bikes with welded bottom brackets and lugged joints because the bottom bracket is hidden behind the crank.
However to the framebuilder, there are some other considerations.
One is water drainage. On a lugged bottom bracket shell, water in all other tubes can flow into the bottom bracket shell without restriction, and a drain hole on the bottom of the bottom bracket shell will drain all water that finds its way into the frame, except for water in the top tube or seatstays. On a lugless bottom bracket shell, there are vent/drain holes, but these do not extend to the edge of the tube, creating little dams that trap water, and in some cases have caused frames to rust through at the joints near the bottom bracket.
The other is that the sockets on a bottom bracket shell do not require precise mitering, or any mitering at all. They also allow for some minor length adjustment. This means on a lugged frame, you can hand file the downtube/headtube miter without concern for length, and you can keep going at it until you get a good fitup. If the miter is off, you can just file another millimeter off until you get it right. The downtube/bottom bracket joint can be cut after to length. The seatube no longer needs a good miter. Only the top tube requires a good miter on both ends, but even then, it will be hidden by the lugs and the top tube/seat tube joint is not as stressed (if sacrifices must be made). Chainstays can also be slightly adjusted for length in the bottom bracket sockets in order to get better wheel alignment. The sockets will allow for minor dropout adjustment in all axes. The tubes can also be twisted in the sockets to allow for angular adjustment.
This differs with lugless bottom brackets. The miters must be tight, which means four more precise miters and joints per frame. Not only that, but these miters must be in precise alignment both in distance and angle relative to the miters/dropouts on the other side of the tubes. This makes it harder to miter by hand, not because you can’t get precise miters by hand, but because you can’t just keep filing things down more if things aren’t quite right, and there are more things to get right.
Kuromori 