Secondary offices are a relic from a time when senators didn’t have any office space at all. At first, the part-time Senate worked on the Senate floor. Everything they needed was at their desks, from writing space to inkwells and even tobacco spittoons.
Over time, the Capitol complex expanded. New office buildings provided space for ever-expanding staffs. Agencies that had been housed in the Capitol, like the Library of Congress and Supreme Court, moved to their own buildings.
As space became available, the most senior senators were given office space, doled out in strict order of seniority without regard to party affiliation. The completion of the Capitol Visitor Center allowed for enough space that every senator now has their own hideaway.
Source: minnpost.com
