In the Center, time of day makes a difference, and so does traffic flow. With less traffic flow, sectors will be combined, and the high altitude area around BFF will go from 134.57 to 135.02, because it's combined with the next sector to the east, and 135,02 has better coverage over the combined airspace (there's only one main/standby set for 134.57 at Cheyenne (CYS), with 1 BUEC (Back Up Emergency Communications) at Crawford, NE.).
135.02 has the Crawford (QHX) site, a site at O'Neill (ONL), one at Rapid City (RAP),and backups at Ainsworth (ANW) and Sundance (SUC). On the mid, 135.02 is used for the Rapid City, Casper and Crazy Woman areas, as well, where 133.67 would be used during the day. This gives coverage for high altitude aircraft from Rock Springs (OCS) to CZI to RAP and east, down to ONL, to Denver (DEN) (you probably knew that one), and back to OCS.
It's a matter of convenience for both the controller and the pilot. Combined airspace means fewer frequency changes, fewer frequency changes means fewer things that can go awry.
Disclaimer: Frequency antennae, in spite of their identifiers, are not necessarily at or near their associated airports. Most are near associated VORs.