Roughly nine out of ten forex trades have a dollar leg, because the dollar is the vehicle currency that connects everything else. A Thai importer paying a Chilean exporter will usually go through dollars rather than trading baht against pesos directly.
For a retail trader the practical consequence is cost. Dollar pairs such as those in major-pairs quote inside a pip during active hours, while a cross-rate built from two of them inherits both spreads.
Example: EUR/USD shows 0.2 pips of spread and GBP/USD 0.5 pips at the same moment, while EUR/GBP shows 1.1 pips. On a standard-lot round trip that is roughly $2, $5 and $11 respectively.
Related: cross-rate, major-pairs