What a will does
A will is a legal document. It names who inherits your property, appoints executors to carry out your wishes, and — crucially — can name guardians for minor children. To do its job it must meet your jurisdiction's legal requirements for signing and witnessing. Nothing in a planner replaces that.
What a planner does
A planner carries everything a will does not: the phone numbers to call, the accounts to close, the subscriptions to cancel, where the spare keys are, what music you'd like played, the letter you never got around to writing. It is the practical and personal companion to the legal instrument — the working manual your family actually opens in the first difficult days.
Using both together
Keep them consistent and keep them together. Your planner should note that a will exists, where it is stored, and who the executors and any attorneys are. Review both once a year: accounts change, people move, wishes evolve. Treated as a pair, a valid will and a well-kept planner cover almost everything your family will need.