‘And it came about, as he was praying in a certain place, that when he ceased, one of his disciples said to him, “Lord, teach us to pray, even as John also taught his disciples.”
The disciples constantly saw Jesus at prayer. Possibly it would be a little unfair to judge their own prayer lives on the basis of silence (one must hope so), but we read little of such prayer. Here, however, we have the indication that they did pray. And indeed Luke does emphasise that all must pray. So Luke's purpose in pointing constantly to Jesus as praying (Luke 5:16; Luke 6:12; Luke 9:18; Luke 9:28; Luke 11:1) is to bring out His close relationship with, and dependence on, His Father.
Now His disciples, through one of their number, express their concern that they might learn to pray better. They knew that John the Baptiser had taught his disciples to pray. They too wanted to learn how to do so.