Longbourn estate is entailed on the male line. Mr. Bennet, Longbourn’s current owner, has no sons. Therefore the Bennet daughters must marry or face a lives of poverty following their father’s death.
And of course, if Mr. Bennet did have a son, that son would be delighted to financially support his mother and five sisters for the rest of their days. John Dashwood certainly was.
The problem with all of this is that Mr. Bennet’s intention to father a son does nothing to actually solve the problem of what to do for his wife and daughters following his demise and while one certainly hopes that a young Mr. Bennet would lovingly provide his mother with shelter until the end of her days, but what if a spinster sister survived him? Mr. Bennet would then be placing his trust in a hypothetical party who was even further removed from himself.
Of course, part of that problem was to be avoided by having the son join with his father to cut off the entail. Something which was possible to do with the heir’s approval and could result in a more equitable distribution of the assets, possibly through actions such as selling off or mortgaging parts of the property (something the entail was designed to prevent as it would decrease the estate’s overall value and thus the family’s prestige) or selling off some resource attached to the property. All of this would have the impact of decreasing the son’s income, possibly limiting his own ability to later provide for his own children.
I could go on, but the point of all this is that Mr. Bennet’s grand scheme wasn’t so much a plan of action as it was a plan to push his responsibilities onto the next generation in hopes that his heir would be willing to bear them.