Today is the transgender flag day, commemorating the creation of the transgender flag by Monica Helms on August 19, 1999. This is the original flag that she donated to The Smithsonian in 2014.
We like flags, man. Flags are cool. Besides, it isn't just a western things, flags are a human thing. Almost every culture out there uses flags and/or banners.
Flags are both literally and figuratively a banner for people to match under. When people protest flying a flag above them is an effect way to ensure anyone who sees them understands their motivations.