Was reading about the new vertical pad loading system SpaceX wants to build. Of interest was this quote
Known as a mobile service tower (MST), SpaceX has managed to avoid the need for the expensive, complex, and extremely unwieldy infrastructure for the first decade of Falcon 9 and Falcon Heavy launch operations. Instead, SpaceX has designed its launch vehicles around the concept of horizontal integration, meaning that its Falcon rockets can be entirely integrated and prepared for flight before going vertical for launch. This approach has ensured easy, cheap access to the entire rocket and payload up until the last few days of static fire and launch operations, lowering the cost of launch.
Beyond Russian spaceflight operations, SpaceX, and a handful of other companies around the world, nearly all other major launch providers and space agencies – including the United Launch Alliance (ULA), Arianespace, ISRO (India), and CNSA (China) – rely almost exclusively on vertical integration. With its new Pad 39A mobile tower, SpaceX will soon join that small club, giving it the ability to compete on completely even footing with ULA and others for lucrative military launch contracts.
The article never addresses why you would need vertical vs horizontal loading. The horizontal method seems to have been quite successful, so why would you need (or even want) vertical?