Linked from
The 11 pages that link to Bounded function, each with the reason it gives.
Riemann integralRelated: Boundedness keeps the sums finite and is part of the standard definition.
IntegralRelated: Boundedness helps guarantee Riemann integrability on a closed interval.
Supremum normRelated: Boundedness makes the supremum norm finite.
Riemann sumRelated: Boundedness is part of the standard criterion for Riemann integrability.