Its surely not coincidental that Jesus explained why He spoke in parables in response to a question from His disciples, after this particular one. For it is not necessarily easy to understand but it is important.

Lets look at His general explanation of parables first.

The disciples came to him and asked, Why do you speak to the people in parables?

He replied, he knowledge of the secrets of the kingdom of heaven has been given to you, but not to them. Whoever has will be given more, and he will have an abundance. Whoever does not have, even what he has will be taken from him. This is why I speak to them in parables:
Though seeing, they do not see;
though hearing, they do not hear or understand.
In them is fulfilled the prophecy of Isaiah:”
You will be ever hearing but never understanding;
you will be ever seeing but never perceiving.
For this peoples heart has become calloused;
they hardly hear with their ears,
and they have closed their eyes.
Otherwise they might see with their eyes,
hear with their ears,
understand with their hearts
and turn, and I would heal them.
But blessed are your eyes because they see, and your ears because they hear. For I tell you the truth, many prophets and righteous men longed to see what you see but did not see it, and to hear what you hear but did not hear it. (Matthew 13:10-17)

Parables then, are not simply good moral stories but contain the secrets of the kingdom of heaven. This is confirmed by Wheaton in his discussion on parables:

It was the mistake of Jülicher to reduce the messages of the parables to moral platitudes. More recent scholars have rightly recognized that they formed part of Jesus proclamation of the kingdom of God.

And further on:

Many of the parables of Jesus are specifically related to the kingdom of God (e.g. Mk. 4:26, 30), and in general the parables are related to its nature, its coming, its value, its growth, the sacrifices it calls for, and so on”

The parables confirm that we all must enter Heaven in the same way, by believing in Jesus and not through works. (e.g. The Parable of the Workers in the Vineyard – Matthew 20:1-16) But they also reveal that not all Christians are the same.

Nor are our rewards, although that is not the subject of this parable. This is hard for those who live in democratic societies to understand and accept, as we tend to apply a completely wrong principle of egalitarianism to all those in the Kingdom. Extrapolating the one man, one vote principle of democracy to a heavenly, all floating about on the same sized, fluffy clouds! Definitely not the kind of Kingdom the parables, or indeed the whole Bible reveals!

The New Bible Dictionary, (Wheaton, Illinois: Tyndale House Publishers, Inc.) 1962.