FTBLib is a library mod. Like other library mods, it contains several coding elements commonly used by a specific list of mods. This saves data storage space by making those mods refer to this mod for needed input rather than putting the same redundant code into all the mods in a given group. This library works for all FTB-related mods, including FTB Tweaks and FTB Utilities. It also contains some code for CommandScripts and XPTeleporters, which are also great mods if you want to create a Factions server with heavily modified gameplay.
Because all four mods listed here refer to FTBLib for common code, you will need this mod to make those mods work. That’s how library mods function. In the case of the XPTeleporters mod, this is an excellent idea because it can give players new ways to travel around their world without giving them the broken /home and /or warp commands to use as they please. The effects here work in single-player just as well, so even if you prefer to play alone, you can still take advantage of all the benefits brought to the game by FTBLib and the mods which depend on it to function.