The disciples' question shows their lack of understanding. They knew God provided manna in the wilderness during the time of Moses (Exodus 16). They were now with someone much greater than Moses (see Mark 9:5–7). They had recently seen him feed more people in a similar situation (6:30–44).
But they still did not understand (compare 6:52). Even so, Jesus used them to provide physical and spiritual food to the hungry (8:6–8). What they could not do alone, the powerful Son of God did through them.