the end, finish
畢 centers on reaching a final point: the end or finish of something. The idea is completion, bringing a process to its conclusion.
畢 originally depicted a hand holding a net, used for catching things, which later extended to the idea of finishing or ending. The modern form is highly stylized and the net origin is not visually obvious.
The top looks like a hand holding a net, and the bottom 田 suggests a field. Imagine finishing a day of netting in the field: the work is done, it's the end.
For ヒツ, imagine a hitsu (a made-up word for a final stamp) marking the end of a document: hitsu -> ヒツ, and the stamp finishes the task.