Flight Numbering

I was looking up my upcoming flight and came across very bizarre (in my mind) info. According to flightmapper.net UA flight UA635 is listed as serving the following routes:
• McCarran, Las Vegas (LAS) - Cleveland (CLE)
• La Guardia, New York (LGA) - Denver (DEN)
• Denver (DEN) - Sky Harbor, Phoenix (PHX)
• Sky Harbor, Phoenix (PHX) - O'Hare, Chicago (ORD)
• Philadelphia (PHL) - Seattle/Tacoma (SEA)
• Philadelphia (PHL) - O'Hare, Chicago (ORD)
• O'Hare, Chicago (ORD) - Seattle/Tacoma (SEA)
• Baltimore (BWI) - Seattle/Tacoma (SEA)
• McCarran, Las Vegas (LAS) - Newark Liberty, New York (EWR)
• Newark Liberty, New York (EWR) - San Jose (SJC)
• La Guardia, New York (LGA) - Seattle/Tacoma (SEA)
• Seattle/Tacoma (SEA) - Newark Liberty, New York (EWR)
• Newark Liberty, New York (EWR) - San Antonio (SAT)
• Pittsburgh (PIT) - George Bush, Houston (IAH)
• Tampa (TPA) - O'Hare, Chicago (ORD)
• O'Hare, Chicago (ORD) - San Jose (SJC)
• Dulles, Washington (IAD) - Albuquerque (ABQ)
• Des Moines (DSM) - San Francisco (SFO)
• La Guardia, New York (LGA) - San Francisco (SFO)
• Ronald Reagan, Washington (DCA) - Denver (DEN)
• Denver (DEN) - San Francisco (SFO)
• Orlando (MCO) - Sky Harbor, Phoenix (PHX)
• Pittsburgh (PIT) - Denver (DEN)
• Newark Liberty, New York (EWR) - Denver (DEN)
• Des Moines (DSM) - Denver (DEN)
• Denver (DEN) - San Jose (SJC)
• Boston (BOS) - Newark Liberty, New York (EWR)
• Newark Liberty, New York (EWR) - Cleveland (CLE)
• Orlando (MCO) - Denver (DEN)
• La Guardia, New York (LGA) - O'Hare, Chicago (ORD)
• Orlando (MCO) - San Jose (SJC)
• Pittsburgh (PIT) - San Jose (SJC)
• Orlando (MCO) - O'Hare, Chicago (ORD)
• Baltimore (BWI) - O'Hare, Chicago (ORD)
• O'Hare, Chicago (ORD) - Denver (DEN)

It looks like this flight number is assigned to a different route almost every other day. I am just curious if someone can explain the logic behind this? Why would a single flight number be used on so many different routes?