In a nutshell
Decimal odds are widely used online and around the world, because they are easier to understand than traditional fractions and provide more betting options.
Why are Exchange odds in decimals?
1) Easier to understand
Which is the bigger price - 8/13 or 4/7? Even if you are familiar with bookies' fractions, then it might take you a while to work out which is better. With decimal odds, you would instantly see that 1.61 is bigger than 1.57.
2) More options
Decimals allow smaller increments and better competition for prices. For example, instead of 2.5 (6/4), you could ask for 2.52, 2.54, 2.56 or 2.58, which are all available on the Exchange before the next standard fraction of 2.6 (8/5).
3) Easier to work out return
Fractional odds represent the profit - 5/1 means you will win five pounds for every one pound staked - whereas decimal odds represent the return - 2.2 means you will receive £2.20 for every £1 invested, including your stake.
So, think of decimal odds in monetary terms. [5.5] means a £5.50 return on a £1 bet, [4.3] means £4.30 and so on.
You will always sees the fractional conversion here on Betting.Betfair when you roll over the decimal odds but some examples
[1.2] is 1/5
[1.5] is 1/2
[2.0] is evens
[2.5] is 6/4
[3.5] is 5/2
[4.5] is 7/2
[9.5] is 17/2