The main reason I love the Energia launch system.
The Energia can be used by itself as a heavy SSTO launcher. The boosters can be fitted with a second stage and used as TSTO medium launchers. The main rocket plus boosters can be used as a super heavy launcher. It can carry a payload either on top or piggyback.
This system had it all. A modular adaptable system that could be used for every type of launch; satellites, deep space probes, unmanned supply craft, manned capsules, shuttles, large space station modules, etc.