The disciples approaching said to him. From the words of Matthew it is evident, that the disciples did not merely look to themselves, but wished also to consult the benefit of others. Being unable to comprehend the parable, they concluded that it would be as little understood by the people; and, therefore, they complain that Christ employed language from which his hearers could derive no profit. Now though parables are generally found to illustrate the subject of which they treat, yet the uninterrupted course of a metaphor may lead to obscurity. So then Christ, in delivering this parable, intended to wrap up, in an allegory, what he might have said more plainly and fully, without a figure. But now that the exposition is added, the figurative discourse has greater energy and force than if it had been simple: by which is meant, that it is not only fitted to produce a more powerful impression on the mind, but is also more clear. So highly important is the manner in which any thing is said.

Christ, in these verses, gives a "reason" why he used this manner of instruction. See also Mark 4:10-12; Luke 8:9-10.

And the disciples came, and said unto him,.... Not the twelve only, but others that were about him, as Mark says, who also were his disciples, and believed in him: these, when he was alone, came nearer to him, who, whilst he was preaching to the people, were at some little distance from him, either in the ship, or on the shore, though within the hearing of him, and addressed him after this manner;
why speakest thou unto them in parables? not that this way of speaking was new and surprising to them; but because it was not easily understood, especially not by the common people, without an explanation, which, as yet, Christ had not given: and indeed the parable was not understood by the disciples themselves; who put this question, not only for the sake of the multitude, but for their own also, hoping to be favoured with the meaning of it.

And the disciples came, and said unto him--"they that were with Him, when they were alone" (Mark 4:10).
Why speakest thou to them in parables?--Though before this He had couched some things in the parabolic form, for more vivid illustration, it would appear that He now, for the first time, formally employed this method of teaching.

Given to you to know the mysteries of the kingdom. Truths that the uninstructed multitude are not yet prepared for. Every one knows that the lessons given must be adapted to the state of the pupil. Spiritual preparation is needful to understand the deeper spiritual truths (1-Corinthians 2:6, 1-Corinthians 2:11).