This is a bit of a generic question since it can be specific to the platform and even to the method. It really comes down to math and getting a value between 0-1. Since 255 is the maximum value, dividing by 255 expresses a 0-1 representation.
Each channel (Red, Green, and Blue are each channels) is 8 bits, so they are each limited to 256, in this case 255 since 0 is included. As the reference shows, systems typically use values between 0-1 when using floating point values.
http://en.wikipedia.org/wiki/RGB_color_model
See Numeric Representations.
These ranges may be quantified in several different ways: From 0 to 1,
with any fractional value in between. This representation is used in
theoretical analyses, and in systems that use floating point
representations. Each color component value can also be written as a
percentage, from 0% to 100%. In computers, the component values are
often stored as integer numbers in the range 0 to 255, the range that
a single 8-bit byte can offer. These are often represented as either
decimal or hexadecimal numbers. High-end digital image equipment are
often able to deal with larger integer ranges for each primary color,
such as 0..1023 (10 bits), 0..65535 (16 bits) or even larger, by
extending the 24-bits (three 8-bit values) to 32-bit, 48-bit, or
64-bit units (more or less independent from the particular computer's
word size).
