Luke 9.

13 He replied, You give them something to eat.

The situation before was that the Apostle came back after Jesus send them out to preach. Before hand the power over evil spirits and to heal sicknesses were given to them. In verse 10 it is described that they start to tell Jesus about all they have done. It seems that Jesus did not reply them but withdraw them with him and went alone(try to) to a city.
Before the disciples made themselves available for others but in this situation they forgot and wanted to send the crowd away.
Jesus maybe wanted to see how the disciples would react when they face a problem they did not face before. Maybe they were very happy about the work they have done and succeeded in carrying out the mission. Jesus knew that they have nothing to give the crowd but they should have the knowledge and ability to make this problem be solved through God.