As a body approaches the local speed of sound the drag begins to increase dramatically. The use of swept wings can delay this drag rise because the airfoil presented to the airflow effectively has a reduced thickness to chord ratio. (The effective chord line is longer).
High aspect ratio wings are used in order to reduce the strength of the vortex at the wing tip that is generated by high pressure air escaping from below the wingtip to the lower pressure region on the top. This sets up a big spiral of air which takes a good deal of energy (sorry, no numbers!) to sustain. This energy manifests itself as drag which directly translates into poorer fuel consumption.
High aspect ratio wings are used where drag must be minimised.
In the case of the U2/TR-1 in order to increase the range.
In the case of gliders to enable the maximum time in the air.
In the case of a turboprop, to reduce fuel use and improve the economics of operation.
The practicality of increasing the aspect ratio has to be balanced against structural considerations such as the rigidity of the wing etc.