Crosses are priced through the dollar behind the scenes, which gives them slightly wider spreads than the major-pairs. Some, such as GBP/JPY, are known for large daily ranges.
Trading a cross lets you express a view on two economies without the dollar in the middle.
Example: if you think the euro will outperform the pound, EUR/GBP isolates that view; EUR/USD and GBP/USD would both also carry dollar risk.
Related: major-pairs, exotic-pairs, base-currency, volatility