Why was Daniel told to seal the book?
Daniel was told to shut up the words and seal the book because God had set an appointed time for the revelation of end time things to be given to the world, and until that time arrived the revelation was to remain hidden from all creation.
In Daniel 12:1-4, Daniel received revelations about things to come in the end, but he was instructed to stop documenting what he was hearing and seeing and to seal the book until the time of the end. This means the revelation was concealed from all creation until God's appointed moment for it to be disclosed.
That appointed time came around A.D. 96, when John received the same revelation Daniel had been given, but in far greater detail. So what Daniel sealed up was, in a sense, held back until it could be unveiled through John and written down as the book of Revelation. Just as Daniel was told to seal the book, Revelation 5:1 shows a sealed book in God's hand, which is the documented, written revelation of things to come in the time of the end.
So the reason for the sealing was simply timing: the full revelation of end-time events was not meant to be released to the world in Daniel's day. It had to wait for the appointed season, which arrived when John was caught up in the vision and given the fuller unveiling that we now have as the book of Revelation.
The scriptures
And at that time shall Michael stand up, the great prince which standeth for the children of thy people: and there shall be a time of trouble, such as never was since there was a nation even to that same time: and at that time thy people shall be delivered, every one that shall be found written in the book. And many of them that sleep in the dust of the earth shall awake, some to everlasting life, and some to shame and everlasting contempt. And they that be wise shall shine as the brightness of the firmament; and they that turn many to righteousness as the stars for ever and ever. But thou, O Daniel, shut up the words, and seal the book, even to the time of the end: many shall run to and fro, and knowledge shall be increased.
And I saw in the right hand of him that sat on the throne a book written within and on the backside, sealed with seven seals.
From the books
This answer is drawn from these passages in the Mary Banks Faith Library.