The R7 rocket

- mostly to avoid engine ignition in flight & due to having a lot of nozzles you need to fit on a rocket

- this was then kept for Soyuz even after an additional stage has been added that is started in flight

- Soyuz 2V uses a more efficient engine & uses only the central stage

The proton Rocket

- the central tank has the maximum diameter you can ship via rail from the factory to Baykonur

- by mounting smaller tanks around it that are shipped separately you can avoid building an overly long rocket, that could be fragile and unstable

Saturn 1/1B

- basically a stop gap using existing Jupiter and Redstone rocket tankage tooling

- strap 8 Redstone tanks around 1 Jupiter tank and you get the S-I first stage

Delta IV Heavy/Falcon Heavy

- you have a rocket that can launch by itself with smaller payload or by strapping 3 first stages together can launch a bigger payload

- better economies of scale as you can doe more with a single rocket design instead of maintaining 2 separate one (big and small)

- in Falcon Heavy case you can also save all the first stages from more demanding trajectories where you would otherwise have to expend the regular F9 first stage

OTRAG

- make dirt cheap and as simple as possible standardized "rocket tubes"

- strap a lot of them together

- fire and jettison in the right order to achieve orbit (check your staging! ;-) )