From a NASA infographic:
NASA’s two crawler-transporters are unique in the world. Originally built in 1965 to carry the massive Saturn V rocket and Apollo spacecraft from Kennedy’s Vehicle Assembly Building to Launch Complex 39, they continued to support human space exploration taking space shuttles to their launch pads for 30 years.
The crawlers are critical elements to enable future launch operations for NASA’s Journey to Mars. They could also be major players in Kennedy’s transition to a multi-user spaceport with one able to take commercially operated rockets and spacecraft to the launch pad, while the other is being strengthened to handle the Space Launch System (SLS) and Orion spacecraft atop the mobile launcher, a combination heavier than the Saturn V moon rockets the crawlers were designed to carry.