This indicates that the main estimates you are being asked to approve as parliamentarians present only a very partial picture of the government's plan. It contains the budget appropriations you are being asked to approve, but none of the budget initiatives. So it's a very partial picture, since it doesn't include the measures that will be presented by the Minister of Finance on April 16, but you're still being asked you to consider it, approve it, or at least debate it. That doesn't give you a clear picture of what the government wants to do over the course of the year.
That's also why, when you, as parliamentarians, receive the government's budget on April 16, you won't see the same amounts that you will have seen in the main estimates at all, because the federal budget is tabled after the budget appropriations. So the numbers don't match up at all. It would be simpler to reverse the order of things so that the federal budget is tabled in February, for example, which would allow Treasury Board officials to include more budget measures in the estimates and make these two documents comparable or at least better aligned with each other.