Crude is produced where it is found and consumed where the refineries are, and those are rarely the same place. Moving it is a large industry with its own market, its own pricing conventions, and its own cycles, and it is the mechanism that connects every regional crude price to every other one.
Freight matters to a crude desk for three reasons. It sets the width of every geographic arbitrage, so it determines whether a differential between two regions is an opportunity or just a transport cost. It is a cost of the storage trade at the expensive end of the curve. And it is a demand signal in its own right, because tonne-miles rise when trade routes lengthen even if total volume is flat.
The pricing conventions are unlike anything else in the oil market and are the main barrier to entry. Rates are quoted in Worldscale, a percentage of a published notional rate for each specific route, which then converts to dollars per tonne and finally to dollars per barrel using a grade-specific density. Owners think in time charter equivalent, a daily earning figure, because that is what they compare against operating cost.
The fleet is segmented by vessel size, and the segments are not interchangeable because ports, canals and straits impose physical limits. See vessel classes. Route choice is constrained further by chokepoints, a handful of narrow passages through which a large share of seaborne crude must pass.